There's a pretty damn good chance that the next couple DLCs, will introduce underwater combat with aquatic monstrosities. First of all, there's the Harpoon Gun that is still in the Fallout 4 files. Then there is also that Quest icon with a ghoulified whale in it. Then you find those mutated fish, dolphins and sharks lying dead on the shores/harbors. Plus, while playing Fallout Shelter today, after the latest update one of my Wasteland Explorers said something about aquatic creatures being even more deadly now, but tasting just as great. I'd bet that the Far Harbor DLC will introduce underwater content.