As much as I love the variety of mods coming out every week for this game I always notice that nobody seems fond of the sea. I've seen a couple of mods over the years, however they don't seem to do much. I remember in Morrowind there was a massive ocean mod that included sailing, fishing, navigation, finding sunken caves and temples, underwater crypts... as well as a wonderful variety of peaceful and killer fish!
The closest I can find for Oblivion is a mod that makes Slaughterfish more varied sizes. Why is this?

Have you tried Alive Waters? It changes the underwater textures, adds a dozen or so whipwrecks and lots of different fish. Or even better, use a combo of Alive Waters - No Fish and the MMM fish mod (don't remember it's name).

Dunno what you mean by water mods exactly, but managing water in the CS just svcks. You basically have a base water table that can't really be manipulated too much

Uh - there are heaps of water/underwater-affecting mods! All the types of things you reeled off, and then some. I know, because I have most of them right here on the HDD.

edit: Diverse Waterlife is the MMM plugin ^^

I haven't checked recently but I am sure there are quite a few underwater homes on ORE aswell...
http://oblivionsrealestate.com/Find_A_Home/find_a_home.html

They're out there if you look - for example http://oblivionsrealestate.com/Find_A_Home/Underwater_Homes/underwater_homes.html or http://www.tesnexus.com/downloads/file.php?id=29688 and many more.

Semi-ninja'ed (no emoticon for that)

I always wanted a shark mod, or a leviathan mod. I mean, there is sooooo much water around Anvil, surely there would be one sea monster :bolt:

Better Cities Anvil adds a few monsters in the Anvil Bay - I was swimming in that area, and saw a huge dead body down there... with some treasure in it. I didn't kill it, so I wonder what killed the monster - another larger monster perhaps? :shocking:

There isn't really a lot of water in Oblivion, sure you have 2 port based cities, only one of which has a port. But the lack of water compared to Morrowind makes water based mods far and few. Sailing mods have the problem that the rivers are too narrow and collisions occur a lot, plus the fact on Morrowind there was a lot of access to the sea.

Tamriel Heightmaps does not improve the vanilla waters any, but does add a lot more sailable ocean area.

TIE (Tamriel Immersion Experience) adds some Argonians that hang out in the water, including a few permanent residents there, but this overhaul makes a lot of other changes you may or may not want. Try TIE4mods as a possible alternative.

There are also several fixes for water transparency/color/reflectiveness around (Phinix's Waterfix is the one in my load order).

Lynge's "Stories from the Sea" and "Shipwrecks" both provide underwater quest content.