agree with eddiewalker. this is an engineering problem and the first two pieces of info you need are the total weight you will be carrying and the rated weight of the floating platform. if B is less than A, don't do it. if B is close to A, do it VERY carefully on a clear day with helpers with other boats.
i also agree the best and easiest way to unload is to beach the front of the pontoon on or near the shore and use some types of ramps to unload the tractor. make sure the lip where the ramps touch the boat is supported by good enough cross members to support all the weight of the tractor plus ramps plus rider. most pontoons are made of aluminum, and are not designed to carry a point load of 1500 lbs even if distributed over four tires. as you move the tractor onto the ramps, most of the weight will be carried by the lip of the ramps and transferred down to the pontoons by a single cross member. this must either be reinforced or make very sure it is beefy enough to do the job. if you tie the pontoon in close to shore so it can't move in, say, 6 inches of water, as you drive the tractor forward to the ramps, the front of the pontoons should sink down and stabilize into the lake bottom taking all the weight of the unit and minimizing the chance of flipping the whole boat or pulling the rear out of the water. (this is the reverse but similar danger when loading on the other side!) just be sure to clear away the big rocks and make a nice place for the pontoons to settle into the bottom and distribute the load over several square feet.
if the beach is soft sand, you might need to take some long pieces of lumber to drive the tractor on to keep it from sinking into the sand.
when loading or unloading and while the unit is on the boat, consider using 2x lumber under the wheels to help distribute the load.
good luck and definitely post up some pics!
amp