There should be a bladder inside the pontoon. Take it out, locate the hole, and if its small, you can buy a pontoon bladder patch kit. Just follow the directions. If its big, I would try and get a new bladder. I'd hate to be out on the water and have the repair fail. Also, if the failure is at a seam, that can be a little harder to repair.
Creek Company Patch Kit (
LINK)
Creek Company Replacement Bladders (
LINK)
Not sure which exact pontoon boat you have, so couldn't get you the exact page for the replacement bladder, but scroll through that 2nd link if that bladder has a big hole.