Underwater is same as Exploring the Deep from several years ago (
Exploring the Deep - UOGuide, the Ultima Online Encyclopedia )
As others have said there is plenty and they gave a pretty good idea and it really depends on what you want to do.
If you want to play in a group then Shadowguard will typically be the best bet. But you can also do spawns in tram and people will help with those too (no PS but you can still get the drops like Lt Sash).
Like
@celticus - I've found myself doing some stuff I haven't done in a while (because it's fun in spurts but kind of boring in large doses) like treasure hunting, fishing, pirate hunting, BOD filling, and other solo type stuff that is pretty mindless and go at your own pace.
Others seem to like the break from the dynamic events (which just ended) but I kind of enjoy having them as an option to sprinkle in. The dynamic champ spawn usually has a 30min cool down from when the boss dies which is nice because you can go do a few t-maps or whatever for 20-25min in between and then hit the spawn.
Long story short there is a ton of stuff to do. If you want more ideas just give a shout in Gen Chat. People are doing all kinds of stuff and half of them wouldn't mind help.