You wont be able to drive with the bed filled with water. My friends did that for senior sunrise at my school. When we drove the trucks to school we had to idle it all the way. and your truck will sag a lot. but it wasnt hard to seal it. just get the big pieces of plastic from home depot or get tarp

 

freakin brilliant

 

Big tankers full of water mitigate the slosh with vertical chambers, like the tank is full of vertical pipes, that way when you hit the brakes or make a turn you have a hundred tiny sloshes. If you made a hard cover with like a ton of short pieces of PVC so it went like half deep into the bed it would drive like hauling a huge load of gravel instead of sloshing around and out of the bed. Helper springs too!