When we bought our '95 Party Cruiser, this is what it looked like on our maiden voyage:
--the right rear was almost sunk!
When I got it home, I removed the plug on top of the log, inserted a small hose and siphoned the water out. I timed how fast it was running out, and calculated there was 90+ gallons of water in the log.
Turned out to be easy to find the leak, it was right at the back, a weld on the edge of the M bracket had pulled up:
The hole was near the top of the log, somewhat covered, and only big enough that maybe 3 toothpicks would fit. But when underway, lots of water flows over the top of the log. Amazing that it could take on that much water. Best I can figure it was probably like that since new. (we bought it in '05, and this is the boat that led to our SkipperLiner)
Rather than go to the expense of welding, I mixed up some JB weld epoxy and packed it in and around the hole. It sealed it just fine, and the plug still 'whooshes' when I remove it so it's holding air! (btw, we still have this boat...)
I have no idea what your pontoons look like or how they're attached, but check along all the welds. If you can't find a leak visually, you can pressurize them (lightly) and spray soapy water over the 'toon till you see bubbles...
1988 SkipperLiner 53x14
1995 Tracker Party Cruiser 32 *for sale*
2003 Chaparral 260 SSI
2000 Allegro Bus 40' DP