Trailers are rated for specific gross weight. it doesn't matter if that is water, food, clothes or bicycles. Full tanks, depending on where they are located, will affect the WD adjustment. The only other downside is the extra weight that you don't need to waste gas pulling if you know you are going to have water where you are going.
2013 F250 XLT
I now have a Keystone Outback, but I try to help when I can.
Bitter Gun Owner