There are apparently SEVERAL suits of power armor in the water. Also found some cases with basic stuff like ammo and caps in it.

When I was travelling, I activated a radio tower and got a few distress beacons. One of them was a nautical distress beacon. I was expecting something underwater that had managed to stay dry inside. What I found was a fishing boat with nothing neat inside. So, Bethesda seems to have ignored the ocean, and didn't put anything cool in there once again. There's some neat stuff on a few islands and a couple of empty sunken ships, but nothing more than that.

while not technically underwater, there is a partially submerged wrecked boat off the coast from the Oceanological Society building that has a key in one of the tool boxes, carl's key or something like that, not figured out exactly where it fits though.

The first DLC should have a major nautical and underwater theme. Use the harpoon gun hidden in the code, add in living radolphins, add underwater caves and complexes ....