ok thanks. so marines dont really need to swim, but im pretty sure an unarmored spacemarine, or scout infantry can swim.

I think the general consensus is "They know -how- to swim, but likely suck at it."

Space marines fight in different environments.
Like toxic,high G, high pressure or even vacuum.
In short where human being without special suit will die cos power armor is already armored space suit.
So here will be no problem to adapt their suit for underwater warfare.
Actually bolter round could be easily adapted to something like small short range torpedo,
by exchanging rocked engine to some kind of pressured gas one.

Ever seen Fallout 4 power armor in action under water