In FO3 you could not attack or even wield a weapon while swimming or underwater.
No, which makes some sense.
However if there was a larger lake monster, something along the lines of a Behemoth in size and HP, if you set foot in the water it might follow you on land where you can fight it...
I dunno. I wouldn't be opposed to the idea, as long as it didn't turn into a fantasy dragon-fighting type thing.
I hate that crap.