how about a semi-submersible barge, it could help distribute the weight better than bladders would. Say something 16' x 16' plywood and 2x6 designed kinda like a fat catamaran, position is so its over the log and flood it down, hook up the chokers and then pump it out. A couple of them might do the trick. 16x8x2= nearly 8 tons of water. hmmm 150 tons eh I suppose that is a lot of plywood and framing. If you had higher water, say 3+ you might go 16x8x4 on the pontoons and get more lift out of it. Just throwing out some ideas. The wider you go tho the more stress on the connecting beams, maybe you could rent some steel from the local yard or something.
One question I've been wondering about for a while is whats the drying process on something like that, must be one long slow process if you want anything finish out of the deal.
oh and that chain saw snorkel, the quarry industry uses an open chain design where the chain itself doesn't always ride in a guide like on a typical chain saw, the Egyptians came up with it long long time ago, they would impregnate a copper cable with silica and attach two wooden handles to each end. Then simply chisel a straight groove to start the blade and have at it with the hand chain. The also used a wood wedges soaked in water trick but thats not going to help us on this one. If its water loged enough the wood should be pretty soft, biggest problem might be pinching the blade. You could set up a smaller barge, same configuration and run a chain between rollers powered by whatever. long enough chain and a few other gismo's and you might just have a pretty slick way to slice and dice without getting your feet wet.
anyway just throwing out some ideas
best of luck with it
how about just a platform of similar dimensions 16x8 each side connected with a cross beam over the log, Use the bladders under it so you end up with lots more bladders spread out over a wider area