I see what you're going for here, but you can't really compare something as bulky as a NEWT suit to something that probably weighs a lot, lot less, in this case, Power Armor, what is it, a Max of 75 pounds? That bulky NEWT probably weighs twice that at least.
I mean, you couldn't even hold anything reliably in the NEWT, however, you can wield guns with Power Armor. So, really, I don't see Power Armor being that absurdly bulky.
Yes, swimming in it would be nearly impossible, but you have to realize, A. It's a video game universe,(Also, Realism, in video games? Madness! Seriously, don't get me started on this.) and B. We don't have power armor in real life, maybe it affects your muscles in a way so you can actually swim in it? I don't know. Seems like advanced Tech can do better than a NEWT.
Fun fact: You can actually swim in lava. There's just the whole dying by burning alive thing you need to worry about.
Wait, if an liquid is very dense, like mercury, it allows you to float on it better. Now, I'm confused.
Also, I'd like to say that I'm not trying to drive you mad or anything.