Water is very nearly incompressible, so we can ignore pressure; it will not affect density significantly.

A 12" schedule 40 pipe has internal diameter 11.94", external diameter 12.75". Cross sections are about 112 in2 (water) and 15.75 in2 (metal). It weighs about 54 lbs/ft. [This much from tables on the Internet] The water inside it is 1344 in3 per foot (that's 22 l, 5.8 US gallons, or about 48.5 lbs.).

So your total weight (pipe plus water) is about 100 lb/foot or 1 ton per 20' stick (this is easy to remember and accurate to within 5% which should be close enough). A bit less than half of that is the water.

-RD