Water is not a solid and will leak through even the best duct tape job especially under pressure. I you want to try to support the inside of the pipe, use tightly packed sand and try welding caps on the ends.

 never said it was what I said was "this will act as the pipe being solid and help reduce flattening".  I used sand first which was a PITA and sloooooooow, then tape and water worked fine. meh whatev I use a real bender now so fill it with unicorns and sunbeams for all i care