ORIGINAL: dfn_doe
Lets real in the dreaming a little bit here guys. There is no way you are going to implement watercooling AND save weight. The negligable ammount of weight you'd reduce by eliminating cooling fins would be replaced by the weight of the waterjacket and fittings easily. Then the weight of coolant lines, coolant, a pump, and a radiator is all going to be extra weight. In the end you may get more stable temperatures, but it is unlikely you'd get better overall performance since the weight increase would more than offset any power increases...
Even if it weighs more, the goal of water cooling is increased compresssion and lower temperatures (less power loss due to heat inefficiency). If you can generate more power, with not much more weight (definitely 1/2 pound or less) its a possibility.
The problem in my mind, wouldn't be weight, it would be manufacturing costs, as well as durability of the engine case at the water jackets, as well as the connecting rod from the increased compression.