I have Rumare Ring that allow me to breathe underwater, but there is not much things underwater in Oblivion, so how about in next TES or a mod that make an underwater city to explore, maybe add some mermaids in it.

Maybe a quest that involve underwater people, like Star Wars Episode 1, and so the water, lake, ocean, river is not wasted resource in the game.

Water makes burden to the computer because of need high resources, so why not make it worth?

Not a bad idea. Maybe not mermaids though. Perhaps a http://www.uesp.net/wiki/Lore:Sload nursery.

You can live down http://oblivion.nexusmods.com/downloads/file.php?id=9388 - and if you look at the user images, you'll find evidence of a mermaid mod, too.