Could my water pill increase drainage post tummy tuck?

No, your water pill works at the level of your kidney to remove fluid from your blood steam.   It will make you pee more but won't affect your drain output.Good question, though!Be patient, your drainage will decrease in due time.  In the meanwhile your drain is doing just what it is supposed to do.

Diuretics make you urinate more and have nothing to do with your JP drainage after tummy tuck. Follow your surgeons instructions carefully.

No, your diuretic will not make your surgical wound drain more.  If you are still having excessive drainage,see your surgeon, and he has to decide whether or  not you have a seroma (if the fluid is clear yellow) or a hematoma and continued bleeding (if the drainage fluid is red).