AB Lammina AL 11.6' water inside pontoons!?

[DougSabbag]

I bought this nice AB aluminum hull, model: Lammina AL, inflatable, in July of 2020, bran new. I suspend it from an aft arch of our sailboat.
Well, this boat has a floor, above the hull, with an open gap between the floor and the hull, at the aft end. So, any water that gets between the hull and the floor, will flow out of that gap when I raise the bow higher than the stern. Then I remove the plug in the transom and that water flows out of the boat.
The problem is that, after aggressively removing all the water that comes out, as described, I still hear water sloshing around inside, it sounds like, in the pontoons!
This is particularly a problem, because there is enough water in there, to make it much harder to get the boat to plane, using the 9.9 hp Tohatsu outboard motor.
When this boat was new, it planed very quickly / easily.
So.... has anyone ever heard or experienced water somehow getting inside the pontoons, or is there another compartment under the pontoons of the hull that water can get into?
And how do I remove this water?!
I'll greatly appreciate any advice on WTH is going on with this problem!
Thank you
Doug