Unless the manufacture states otherwise, the TT was designed to carry water.
I know some folks have had issues with this but that is just poor design on the part of their TT manufacturer.
TT are self-contained unit and all the necessary components run off of the battery or LP.
My TT hold 40 gallons of Fresh water and I always have it at least 1/2 to 2/3rds full.
It is true that the water adds about 300 lbs. to the TT weight but my tank is directly over the axles and with a loaded for the road TT weight of 6200 lbs. I never noticed the difference.