Water cooler running off of a 110V plug like your computer or the 12V PSU in your computer?
77W CPU + 260 for the GPUs + 50W for other stuff ~ 500W. Depending on how high/aggressive you are on your OCs you could be looking at 600-650W. 750W will do fine, more if your water pump runs off of the computers PSU. How much more will depend on the pump.
The water cooling system is just a maybe, I don't really need it though so pretend I'm not getting it.
I probably wont OC too high so I'm going to assume the 750w should do the job perfectly!