Grand Seiko Ushio 300 Review: A Smaller, More Accurate Spring Drive Diver

Introduction

Grand Seiko has built one of the most passionate followings in the watch industry through its meticulous attention to craft and detail. Yet two pieces of feedback have followed the brand across social media for years: collectors wanted micro-adjustment on the bracelets, and they wanted a more wearable dive watch that did not exceed 43 mm in diameter.

Last year, the first request was answered with the release of the SLGB003, featuring high-intensity titanium construction with on-the-fly functionality within the clasp. That introduction also hinted at more possibilities with the UFA platform and the 9RB2 caliber, which enabled a Spring Drive movement to fit within a 37 mm case. A smaller Spring Drive dive watch suddenly seemed possible.

Just a year later, Grand Seiko has delivered exactly that with the release of two UFA divers: the blue SLGB023 and the green SLGB025, also known as the Ushio 300s.

Who These Watches Are For

The Ushio 300s are premium offerings from Grand Seiko. The breakthrough of UFA technology is phenomenal in its accuracy and its ability to shrink the presence of the watches, but it comes with the result of UFA being a top-shelf offering, which leads to a higher price.

These watches are unlikely to be someone’s first luxury watch. Instead, they sit alongside the first class of divers in the industry, comparable to the Rolex Submariner, the Glashütte Original SeaQ, and the Fifty Fathoms. The Ushio 300 sits at the intersection of premium dive watch offerings, with elite Japanese craftsmanship and accuracy standards that leave competitors behind.

This all comes while hitting the new core elements collectors were looking for: micro-adjustment in the clasp on the fly, more wearable dimensions, and a more accurate movement.

Design and Build Quality

The Ushio 300s incorporate the common requests into a smaller case on all measurements. The real breakthrough for a Spring Drive dive watch at this size is the result of the caliber 9RB series, which saves multiple millimeters on the diameter of the movement compared to the 9RA series and is significantly thinner than the 9R65.

This leads to the tidiest set of dimensions ever for a Grand Seiko diver: a 40.8 mm case diameter, a 12.9 mm case height, and a 48.5 mm lug-to-lug. In spite of the newly tailored case, the Ushio actually improves in the water resistance department compared to pieces like the SLGA023, delivering 300 m of water resistance instead of 200 m.

The faceted case contours follow the modern Evolution 9 design language, with curved angled lugs featuring mostly hairline brushing and flared bevels showing Grand Seiko’s masterful Zaratsu polishing. The case sides sweep elegantly toward a partially guarded screw-down crown offering fantastic grip and signed with the GS logo in relief.

The Ushio is constructed of Grand Seiko’s high-intensity titanium, which weighs around 30% less than steel. The overall effect comes across as lightweight yet shining with a brilliance that is miles away from more tool-like renditions of titanium. The sapphire crystal gets an anti-reflective coating offering an unobstructed view of the dial, and the 120-click bezel’s dive scale insert gets a lume pip at 12 and is constructed of color-matched ceramic for added scratch resistance.

The bezel has been re-engineered for a lower profile, and the teeth have been refined for better grip. In totality, this watch wears rather true to its size. One of the hidden details of its ability to not wear larger is the dial-to-bezel ratio, with the bezel making up more of the total share, making the dial and therefore the watch appear smaller.

Dial Details

The Ushio 300 has two colorways available with this release: the SLGB023 in blue and the SLGB025 in green. The dial is an evolution of the previously released Ushio pattern dials, getting a beautiful gradation effect. The deep blue variant calls to mind the deeper waters of the sea, while the green suggests the shallows closer to the shore.

Both colors offer dramatic shifts with changes in lighting conditions. The green variant is especially dynamic, as the darker green transforms to an almost silver hue closer to the center as a result of utilizing Grand Seiko’s optical multi-layer coating technique, enabling a wide reflective phenomenon with changing intensities of color in the dial.

Against these remarkable backdrops, the dial elements are worth a closer look. A color-matched angled chapter ring features a minute track printed in white for easy legibility. The sloping minute track allows extra room for Grand Seiko’s jewel-like applied hour indices. The beveled and polished hour markers take a rectangular form rather than circular, as on other divers from the brand in the past, being upsized at the cardinal points and capped off with a double trapezoid at 12.

Grand Seiko makes good use of its proprietary LumiBrite material here, shining exceptionally. Moving further inboard, you will find the applied GS logo under 12 with the printed wordmark just below that. The Spring Drive UFA and the 300-m depth rating are called out in the three lines of text above six. The dial does away with a date complication to allow indices extra breathing room, and the front-facing power reserve indicator rests between seven and eight.

The brushed titanium hands are broad and highly legible, redesigned from those on the original Ushio diver for optimum visibility and aided by the glow from the LumiBrite infill. The minute hand takes a wide arrow stance, with the second hand featuring a lume lollipop counterweight sweeping along only as Spring Drive powered movements can.

Bracelet and Clasp

Last year with the release of the SLGB003, Grand Seiko finally showcased its on-the-fly micro-adjustment. With the Ushio 300s, the brand gets even more bells and whistles appropriate for the dive watch genre.

What remains from the on-the-fly concept is the micro-adjustment system in the clasp, which gets three clicks of micro-adjustment around 6 mm in total, with 2 mm for each stop. To employ the diver’s extension, open up the clasp and depress the extension button to add length in order to wear the watch over a wetsuit. In total, there is around 24 mm possible of adjustment, with 18 of those coming from the diver’s extension.

As one final element of the clasp, when the clasp exposes the red indicator, this means that the clasp is not locked. However, with a simple switch of the button, you can then cover this linear indication to lock the clasp in its secured position.

With the drilled lugs, strap swapping with the 21 mm lugs is going to be made easier in case you want to swap. That said, the bracelet is very complete in its presentation and pairs well with this package.

Movement and Accuracy

No one was asking Grand Seiko for a more accurate Spring Drive movement. So when they unveiled the 9RB2 caliber last year, it was a complete surprise. The Spring Drive caliber 9RB1 is a new version of UFA, built for the goal of achieving a smaller, thinner dive watch, developed at Grand Seiko’s Shinshu Watch Studio in Nagano.

Great care was taken to ensure the 9RB1 was not only thinner to allow for a trim case height, but more resistant to shock to maintain its tool-like watch ability. For instance, the single barrel and magic lever winding system are offset from the center of the movement, reducing its vertical stacked height. The gear train also gets a shift, with its layout being flatter as opposed to stacked, and the power reserve has been moved to the dial side of the movement.

It is an intentional choice, one that allowed the new Ushio to retain a closed case back for greater structural integrity and for helping to secure a trimmer case height. A one-piece center bridge reduces thickness, lowering the overall center of gravity, and a redesigned rotor also improves impact resistance, as does the lack of a fragile pallet fork and escapement wheel in the Spring Drive movement.

All of these factors add up to the smallest professional Spring Drive diver that Grand Seiko has ever produced. As for the UFA technology itself, the movement is more accurate over the span of an entire month than a Swiss chronometer is in a day, or plus or minus 20 seconds per year.

UFA stands for ultra-fine accuracy, a recent Spring Drive innovation first introduced in 2025. According to Grand Seiko, UFA represents the most accurate watch ever made powered by a mainspring.

In the 9RB1 UFA caliber, the movement’s regulation system is entirely vacuum sealed for stability and protection against external forces like humidity, static electricity, and temperature fluctuations. After a three-month aging process, each individual quartz oscillator is measured at several temperatures after the movement is cased, and those measurements are used to program the thermal compensation data into the low-pressure IC inside the regulation system. A sensor inside the IC measures temperatures 540 times a day, using the data collected to compensate for any variance to the oscillator’s performance.

This caliber, as compared to the 9RA series, has a single barrel and will not hit the 5-day reserve, instead offering a not-so-shabby 3 days. However, with accuracy standards that reach all-time levels along with a smaller package, it is far from an insurmountable tradeoff.

Blue or Green: Which Dial to Choose

When it comes to the question of which dial is better, it is always going to be subjective. The green dial is unlike anything Grand Seiko has done to this point in the diving category, especially with the optical multi-layer coating and its dramatic effect. The blue, on the other hand, is the more cerebral choice of the two when it comes to day-to-day wearability and versatility, and still looks fantastic with that Ushio tide pattern.

If you have several Grand Seikos or have another Grand Seiko and are looking for something that is a bit more memorable, not only from the brand but also can separate from the broad landscape of dive watches, the green might be where your eyes go first, despite it might not having the same versatility as the blue one.

Buying Advice

The Ushio 300s represent Grand Seiko’s answer to those looking for what the pinnacle of Japanese dive watches can be, and they did so by keeping in mind the common asks among their fan base: a smaller case, micro-adjustment on the fly, and a more accurate movement.

These are premium offerings, so they are positioned alongside the first class of divers in the industry rather than as entry-level luxury pieces. For collectors who have been waiting for a Grand Seiko diver that wears smaller without compromising on water resistance or accuracy, the Ushio 300s deliver on all fronts.

Conclusion

With the Ushio 300s, Grand Seiko has finally been able to develop an answer to those looking for what the pinnacle of Japanese dive watches can be. The blue SLGB023 and green SLGB025 offer a more wearable 40.8 mm case, 300 m of water resistance, on-the-fly micro-adjustment, and the ultra-fine accuracy of the UFA caliber.

Whether you choose the versatile blue or the dramatic green, these watches represent a significant step forward for Grand Seiko’s dive watch lineup.