There are lots of ways to do it...lots of folks just clip their backpack to the hammock support line. When I'm using a hammock without a bug net, I'll clip it onto the structural ridgeline...then I can slide it to the end of the hammock when I don't need it, and slide it down to the middle of the hammock to get stuff out of it.
Gear hammocks work well, too. Or a Blackbird's shelves, or the Ridgerunner's saddlebags, or a peak bag.
Re: water under your hammock, you might be interested in this thread...