You know what I want in my DLC for Fallout 4, Bethesda? Sea Monsters!
Seriously, it's pretty surprising that we don't often hear what's out in the water. Maybe it's because there's vertibirds to just fly everywhere but I don't know. It seems to me if Bears and Mosquito's could get mutated into monsters by the bombs so could a few whales or something. It'd be easy to come up with some excuse plot too to make it happen. Maybe there's some old institute tech on an island you have to go check out, but it kicks up fierce radiation storms so just flying over to it isn't an option. So maybe you've got to sail there.
Technical limitations maybe. Since it's hard to do giant overland maps and stuff when you've already got one. But make it happen!