Several companies make bladder style tanks for fuel and water. Check BoatUS or West Marine. The fittings on these tanks aren't real big (between 3/8" and 5/8"), so it might take a while to get water in and out. They also make bladder tanks to weigh down the back of ski boats to get big wakes for wakeboarders,
like this .
I've seen tunnel tabs made by taking a trim tab (or two) and adding a plate that spans the tunnel. (Make sure all the tab rams are hooked up to a single pump, not separate ones.)