since water takes the path of least resistance.. I would offer the following unproven suggestions..
If you are going to go with this model.. I forsee the water going pretty much strait up.. and not using the fins.. therefore not effeciently taking the heat away unless you have high volume. If you want to try out this model, perhaps the top fin should be one piece and not 2. Therefore forcing the water to go around and into all the other fins, and denying the path of least resisitance. I would think this would give you some performance gains on your current model.
Thermodynamics are crazy from what I know of them.. the best way to find out is to tweak and try. I encourage you to try every design if you have the resouces and tweak until you find out for yourself what works best. If you dont get the most efficient wb.. then at least you know what works and what doesnt for next time. Unless you have a professional program that can simulate the process..