I have that seat... i plan on using it this weekend. I have had similar seats so ill give some feedback.
I had the old 1800lb fat seat in my boat, that had the arms that went along the engine compartment. When I filled that up with water, my swim platform was almost knee deep. The front of the boat looked like it was a rocket aiming for the sky!
I suggest putting weight up front. If you put that 1200lb seat in the back and fill her up, try to get 800lbs up front. Get a sack under the bow and stick your fattest friend up there and you should be good.
I filled up my fat seat with air when we didnt want the weight in the boat and it eventually popped. We would have 3 people sitting on it and eventually the seams just gave out.
I will be filling up the sofa with air this weekend and giving it a shot... I will let you know