Well we already have some hint as to Suigetsu abilities, and would think it centers around him being a different kind of substance. He formed himself out of a puddle of water, and increased his muscle size instantaniously to weild Zabuza's sword. Even when we saw himabout to fight the curse sealed legion, he was struggling to hold the sword upright.

My guess is that he can control the makeup of his body, muscle structure, execetera, to give himself a temporary increase in physical prowess. I would also think this serves to drain him, as he's not in a bulked up form all the time.

And does anyone else think that if he's out in the sun too long, he'll evaporate? He get dehydrated really quickly.