Did I really burn this much fuel??

Me and my girlfriend went to Ft Myers Fl. for a 3 day vacation. On one of the days we rented a boat to go cruising around the water for the day. The place wanted 200 dollars for a full day or 150 for a half day plus you pay for the fuel you burn. We are on a tight budget so I got the guy to give us a full day at the price of a half days rate. The tank is full when you leave and they fill it up when you get back and you pay for that. The boat was a 17 foot pontoon boat with a yamaha 70hp motor. We were in the boat from 11am to about 4:30pm. About 5 1/2 hours. In that time we stopped twice for about 20 minutes and 45 minutes. So id say the engine was on for about 4 total hours. In that time we were full throttle about 75% of the time. Had a few no wake zones to go through. We get back and he says we burned 29 gallons of fuel!! At 5 bucks a gallon! You do the math.. Not expecting this at the least!! Did we get ripped off? Was the tank not full when we left? I didnt check..