Our marina told me once they do that on a lot of older pontoon boats. They said just let them leak and let the toon fill up with water. Then install a drain plug when the tube gets too full, and pull it out once a year to drain it, or whenever the tube gets too full of water. Obviously today pontoon boat performance is far better than it used to be, with the old boats being not much more than a floating dock and draining a pontoon was only done just before the boat sank. By the way if I couldn't have a boat, a floating dock would suit me just fine if I could set a few chairs and a grill on it. That was the "old pontoon boat owner's" philosophy on life. Of course that's changing with today's pontoon boat being high performance.
Another thing they told me, and this was in all seriousness, was to just drill a hole (1/8th inch), let the water drain, then plug the hole with epoxy or JB Weld. Repeat every year or two (remember mine took 5 years to fill to where it was noticeable, but then again my boat didn't sit in the water all season either). They said they did that all the time. But, if you look at the average pontoon boat in our marina, you'd have a hard time knowing it wasn't a pontoon junk yard, so take all that with a grain of salt. This isn't a marina full of performance pontoon boats.
You know, now that I think back, it's funny they never ONCE said anything about actually fixing the leak! When I told them I was looking for an aluminum welder to fix my pontoon, they told me about a guy that helped them weld/build a marina boat (a work boat with a hoist installed in the center). They never once mentioned they used him to weld pontoon leaks, because they didn't have them fixed! They just drained them and glued the hole shut...
