Well, I don't know about other things, but the second Soul Drinkers book, Bleeding Chalice, has Space Marines in full power armor on boats, and when they're knocked off or the boats are smashed, they sink and drown as they're weighed down by their power armor. In Brothers of the Snake, they dive a trench in the ocean nearly naked (and can live twenty or so minutes underwater, iirc Priad did something like twenty-six or twenty-seven), as well as go out in boats to spear giant water wyrms. No fighting underwater, though. As for scouts... I dunno, when you look at them, carapace doesn't look anything like as heavy as power armor, but I've never read anything that says they can or they can't.
"You all did see that on the Lupercal
I thrice presented him a kingly crown,
Which he did thrice refuse: was this ambition?
Yet Brutus says he was ambitious;
And, sure, he is an honorable man."