I don't think I've ever liked an underwater level in any game so this would really be the "least worst"
Iunno I'm thinking hard here and probably the submarine sections in Red Faction 2 because it's the only example I could think of that isn't a platformer because I find them universally abysmal in any platformer.
I think the best underwater level is about 12 foot deep. You get to swim around in the water freely and you can challenge people to who can touch the floor the fastest.
Just kidding, gotta be all levels from FEEDING FRENZY Because the idea of eating your own kind to make yourself fucking fat is a brilliant idea to commit mass murder.