[Al Skierkiewicz]

Water cooling is way off the list...
If I was judging your robot, I would first and foremost consider water to be an electrical hazard. (damage to robot electronics due to conductivity or to humans from hot coolant) If the cooler required a recirculating pump, the pump would be banned since it is not a kit part (motor). The potential for damage to the floor (any basketball court we may play on) may result from a leak, etc.
However, I have seen some interesting closed loop, convection coolers over the years. These were in sealed (soldered) pipe and some used exotic coolants but they worked. One I know of was used by RCA to cool power tubes in a vintage (1960s) video recorder. The weight of this additional material is still a factor and water is heavy.