Well, the extra weight is one. My jet ski trailer is designed more for newer skis, and if I fill up the skis and all the canisters, the tongue weight goes up at least 100lbs. Hard as hell to lift onto the ball. The boat actually holds over 200lbs of fuel, so that makes a considerable difference. Then you're driving with that much more fuel, all of it sloshing and evaporating. On a hot day, I've had one can blow off the cap. I've also had one of my skis barf gasoline out of its gas cap. Found out that it was cracked that day. On my boat, if I fill it all the way, it will actually spew some fuel out of the vent. I've since learned to go very easy on the pump nozzle for the last few gallons so as to not make a mess. I'm pretty sure that driving the boat with a full tank makes some of the fuel spill out of the vent. I'll find the side a little slick upon arriving at the ramp. I'll fill it up at the best price around town, but not before a 2+ hr drive. For that, I'll try to keep it as empty as possible. But that's just my .02