The bottom of the oceans and rivers are probably not places you've explored very much within Skyrim, I personally never did, but you'd be surprised what treasures can be lurking underneath - and now, this pretty awesome overhaul mod makes it even more worthwhile.
Depths of Skyrim overhauls the underwater environments to make them much more interesting. There's now over 1,000 species of fish, thankfully none of which are particularly violent towards you, as well as loads more flora and fauna.
The best bit though is there's loads more items and hidden treasure to discover, making underwater exploration much more rewarding and less of a chore.
Oh, and there's plenty of horkers swimming around too, which is fun.
There's versions available for both regular and Special Edition, which you can check out here.