It's fun to imagine a vast array of water life. But uh, lets be serious, who will REALLY take the time to explore the depths? There won't be a lot of lakes and such, this is Skyrim. North, snow, tundras. What few lakes and such will be there aren't going to have incredibly varied aquatic ecosystems. A few slaughterfish and maybe some neutral fish. On the ocean coasts have a passive coastal creature and a rocky, craggy ass coast infested with slaughterfish. Seriously.
This isn't the focus of the game and frankly I don't want to sacrifice RAM for a bunch of sea critters I'll only see if I'm an Argonian, and I [censored] hate Argonians.
I take time. Once I realized there were grottos in Morrowind. I always kept one eye to the coast, and I always looked near islands in the water
I'd say: Who will REALLY take the time to explore meaningless, empty depths with a few fish every here and there?
And: Who will REALLY take time to explore depths with secrets, underwater caves, sea monsters, treasures, beautiful environment, different kinds of fish, etc?
Oblivion's waters were a joke.
The focus of the game is what you make it, isn't it?And if you think that RAM is so precious that it can't stand some extra objects, then I think you're either 1) have a 10 year old PC... or 2) have no idea what you're talking about.