Among the water overhauls, Cathedral - Water Overhaul probably has the most crystal clear water, but once you dive in it's the same blurry muck.  This patch makes the water look just as clear while diving.  So you can finally explore the sea.

If you installed a few ocean overhaul only to find out you can't see anything, this is a mod for you.

And here are a few cool underwater mods to fill your ocean floor with.
Sea of Spirits
Depths of Skyrim
Underwater Treasure SSE
Improved Fish SE
Dugongs and Manatees