I fired up my first new local testing world for 1.13 last night, and _wow_ you're right.
The water is packed with new stuff. Honestly, I'm surprised that the game keeps up as well as it does on my ancient hackintosh with the thousands of wriggling kelp blocks within my view.
I saw my first sea turtles on a nearby beach, and they're adorable.
I found my first shipwreck, swam with dolphins, and got attacked by the new drowned zombies all within just a couple nearby chunks of where I spawned.
It quickly made me wonder: how badly will the new drowned zombies break any old mob traps? I think I read somewhere that regular zombies can "turn" when submerged long enough, which presumably means the old drowning-style traps are no good for them.