I can remember spending a long time adventuring under water in Morrowind and also Oblivion. I must admit so far I have nut done a lot of swimming I have found two behind waterfall treasure cavins so far. Apart from that the swimming I have done so far has not inspired me ( I am level 13 ). Any one done much exploring underwater and what are your thoughts?

More fun than Oblivion, less fun that Daggerfall.

After getting my first waterbreathing item I immediately went up north and swam around for an hour or so - apart from some shipwrecks that caught my attention - god it was boring.
The best chance of finding anything useful, or at least a coin or two, three is to stick to lakes and rivers.. my experience says the best chance for some treasure is near waterfalls.

at least there are plants now i suppose. I cant really remember what was in oblivion.
Ohwait you could hunt slaughterfish. LOL well yea that was the only thing? I like riding waterfalls though, thats quite good enough for me.

No item for waterbreathing yet.... Near Bleaks Falls Barrow in the river there is an underwater building. Mainly 1 chest and some barnacles.