Don't forget to take into account not only the pipe weight but if making a platform on top the board weight as well.

Also Bill's numbers are accurate using inside diameter. Just an idea/tip though, if you require more displacement just use two T unions with a pipe down the center. Looks like every ~5' of pipe gives roughly 60 pounds of lift for 6".

Be careful if you try using SDR 35(6.275 O.D.)green sewer drain pipe it is not very compatible with Schedule 40 fittings designed for white pipe(6.625" O.D.) which is what most box stores stock. I feel it will cause headache and heartache when trying to make the floats watertight.

Hope some of this helps or makes sense running on little sleep.