Bedliner brand DualLiner has gone to the trouble of creating a complex infographic explaining why filling the bed of a pickup with water to create a mobile hot tub is unsafe and you shouldn't do it, no matter how "fun it looks on YouTube."
After seeing this I'm a little embarrassed the idea never even occurred to me, but now I can't wait to try it. I mean, with somebody else's truck obviously.
You probably remember from middle school science that water is way heavier than most things a pickup truck would carry. But this little chart is great for figuring out how much heavier... along with a nice roundup of late-model truck bed dimensions and payload capacities.