Weak vacuum in reverse. you don't want to make it too tight because you can damage stuff. Soap down the hulls and see where it is coming from. Most likely it is from the ports themselves. Mine doesn't take on water unless I have capsized and then the water almost definatly comes in through the vent holes on the rear ports. I guess if I were really concerned I would just cover them up with a piece of vinyl tape. If the pressure builds too much then it will still "pop" the tape but not allow any water in when capsized. Other place to check is the beam/hull joints.
After that it is all the through-hull stuff like cleats, etc. If your blade is like mine, it is possible it is coming in through the fittings that keep the track on the hulls where the tramp attaches.
If you are worried about the cheap o-rings in the ports then goop them up a little with petroleum jelly and that should make them tighter. You can replace them with different ports if you want to go that far.