According to back of the envelope calculations I am going to have a high-intensity surface cooling problem. I only have one good thermal heat transfer text book and have limited information about forced water convection heat transfer. That's why I like solar steam so much. I do not know what boiling alcohol would be, could be a possible candidate for heat pipe transfer.
Phase change is a super heat transfer mechanism.
But it seems that optimum turbulent water flow on the other side would require a 78 C difference. If the back side was packed with tubes then the exchange surface area would more than double perhaps requiring 35 C. That would be better than a single rectangular tube. Boiling alcohol might be an alternative. A boiling alcohol heat pipe could be self contained, not difficult to build. I would also like to try a stack of tubes from manifolds passing through and soldered on the back and the sides of solid copper bars for maximum water surface area and flow control, first turbulent pass nearest the hot side.