Definitely!
1) First add lots of fishes. Normal, friendly, passive fishes. Many different kinds of fish.
2) Secondly, there should be coral reefs, seaweed, cool underwater rock structures and UNDERWATER CAVERNS! Perhaps even a few sunken ruins... like in Morrowind
. That was awesome.
3) Add a few hostile fishes and creatures, a few bigger ones (dreugh?) and a few smaller ones (slaughterfish for instance).
4) Add SEA MONSTERS to the deepest depths of water. They should be
rare, deadly, big, incredibly strong and terrifying!
Small lakes and that are fine. You should feel pretty safe. BUT THE DEEP WATERS! THOSE YOU SHALL FEAR!
Then again, sea monsters should be very rare
I can't decide if they should be respawning or not. It leans towards not respawning. Those kinds of creatures are too epic to respawn!
http://peterlarson.files.wordpress.com/2010/07/kraken.jpg
http://www.mythslegendsandfolklore.com/wp-content/uploads/2010/05/Orange-Leviathan_small.jpg
Edit: I think that if sea monsters are non-respawning, then they shouldn't always be on the same place. Perhaps this can be done rather easily by giving them good AI, so you never encounter them on the same spot... and therefore can't prepare yourself as much