While I was swimming in the lake, I was near the limit of my encumbrance and shouldn't I drown or get weighed down? For power armor I think that they should be able to walk underwater and have a rebreather attached to it already.

I looked into this for my Powered Power Armor mod, it adds a rebreather upgrade. Couldn't find a way to do it though. I remember reading somewhere that ghoul reavers don't swim, maybe somebody that knows more about it than me could take a look at them in the geck and see how they differ from everything else.