Every other watch in Seiko's lineup has been treated to an aesthetic update for 2020, except one: the Tuna. It's remained visually very much like the Tuna of 2019, and that's OK, because the Tuna isn't about its looks anyway. The Tuna's core strength is its incredible technical prowess and genuine legacy as a trusty tool for saturation divers. You know, the kind of divers that go down and live in a compression chamber until their blood and tissue are saturated with inert gasses that allow them to work at depth and decompress only once. This isn't a watch for the we-took-a-vacay-to-the-Caribbean-so-we-went-diving crowd. Instead, it's engineered for the narrow slice of divers who do it for a living. For them, Seiko has introduced two new models: the S23631, rated to 1000m, and the S23629, rated to 300m.