Blue Submarine No. 6

Nearly all of Earth is underwater, and the guy humanity is counting on to help fix that would rather stay home and drink. Tetsu Hayami is a former submarine pilot who walked away from the Blue Fleet, the last real naval defense force keeping human survivors alive. When the situation gets bad enough, he gets pulled back in — and the situation is very bad. A scientist named Zorndyke has spent years engineering hybrid sea creatures designed to wipe out humanity, and his creations are winning. Hayami ends up aboard Blue Submarine No. 6 alongside Mayumi Kino, a sharp and driven officer who has absolutely no patience for his checked-out attitude. The tension between them gives the story something to work with beyond just underwater combat, which there is plenty of. At only 4 episodes, it moves fast and doesn't waste time. The animation is late-90s Gonzo at their experimental peak — they blended 2D character work with early 3D for the mecha and submarine sequences, and paired it all with a jazz-heavy soundtrack that gives the whole thing an unexpectedly cool atmosphere. If you liked the oceanic isolation in Gargantia on the Verdurous Planet, or the military tension in Arpeggio of Blue Steel, this scratches a similar itch in a fraction of the time. It's not trying to reinvent anything — it's a tight, well-crafted action OVA with a genuinely interesting world and a lead character worth watching.