What is the average density or total weight of the pontoon? That's the crucial point. If the pontoon is '30''x24''x25ft' or 30/12 ft by 24/12 ft 25 ft, and can be approximated by a rectangular solid, then its volume is about (5/2)(2)(25)= 125 cubic feet. If the density is
lbs per cubic foot, its total weight is
pounds. Assuming that the 24"= 2ft is the vertical measurement, if it sinks to a depth of d feet into the water, it will displace (5/2)(d)(25)= (125/2)d cubic feet of water.
It will sink down into the water until the weight of the water displaced, that is the volume that is the below the water times the density of water, is equal to the weight of the pontoon. The density of water, at 70 degrees fahrenheit, is about 62.3 pounds per cubic foot. So the pontoon, without any other weight on it, will sink to depth d satisfying (62.3)(125/2)d= weight of pontoon. If the pontoon is supporting an addtional weight, X pounds, then it will "just float" (sink to depth d= 2 feet so its top is just awash) with (62.3)(125)= weight of pontoon+ X so X= (62.3)(125)- weight of pontoon.
Again, you need to know either the average density of the total weight of the pontoon to find either how deep the pontoon itself will sink or what weight it will support.
