I'll echo what the guys have already said. I have a comp with the 351 in it. When I fully sac out my boat and load up the friends to go riding, I use around 6 GPH depending on how much starting and stopping I am doing. You'll probably find that the 454 will do better under load than the 351 in regards to gas use due to the additional HP and torque available. I will add the caveat that I motor out to where I plan to ride and then fill up ballast and then empty as soon as I am done before I head home so my fuel numbers might be a little off compared to others.
'86 Comp TS6M
Monster Tower
1,500 lbs Ballast
Riding Hyperlite Boards