In morrowind outside seyda neen I was attacked by slaughterfish. A guard came running, went underwater to get the fish, and he was stuck somehow and drown himself. I got a neat imperial armor and a silver spear at lvl 1
But over to topic : Add more fish types underwater, make the water more clear. More shipwrecks with valuable loot. But also make the seas around Skyrim much more dangerous and scary. If the player want good loot he must face ancient creatures deep beneath the waves. In lakes and so on, make more caves. And in some of them the native vampires in Skyrim.
Guards, well not much to say other than make them act more realistic. They are not psychic police -___-