Is nearly 6 gallons of gas excessive for a 50 HP outboard to burn in less than 2 hours?

I have a 1985 Mercury 2 stroke 50 HP outboard on my 24' pontoon boat that weighs less than 2000 lbs. Last night I was out with one other adult for under 2 hours. For two thirds of the time we were at "no wake" speeds, with the remainder of the time at full throttle. I just got the boat so I'm trying to figure out if this is the typical gas consumtion I should expect, but it just seems excessive to me. I've experimented with the trim and the prop is a little banged up, but not too bad. We were out in an inland chain of lakes with just a little chop. The motor starts, runs, sounds and shifts like a champ. The only other thing I can think to mention is that the boat seems slower than I anticipated, and when I'm at full throttle, the motor seems like it should be revving a bit higher. Can anyone clue me in?? Thanks!