Oh, and BTW, those Tuniq Tower 120s are great for cpu cooling, but they work so well because they are intended to transfer alot of heat (90-120watts worth) from a small area. I dont know if they would be so great for other applications though.
Heat pipe based coolers require a certain minimum temperature for the phase exchange process to take place which makes the heat pipes work. Go under this minimum, and you are going to see worse performance than a regular heatsink with just alot of copper and fins. Heatpipe CPU coolers are great on CPUs, but even at that, you can tell that they have certain minimums to work because their performance when a CPU is at 'idle' (less heat) is often worse than non-heatpipe coolers. Their real benefit isnt seen until you test the CPU under 'load' (more heat) and then the heatpipes contents starts to move that heat away very fast.
So for cooling other things, like pumps, its in your best interest to make sure you have enough heat for a heatpipe cooler to work... or its just an expensive waste.
I would suggest looking at some of the older cpu coolers that arent heatpipe based. They are much cheaper, and with most pumps, you dont need heat removal from such a small area...the reason for all the expense.
If you do go the heatpipe route, I would not suggest the Tuniq 120. Too much copper near saltwater... it wil corrode pretty fast in humidity (if its a concern). The CNPS-9500 AM2 cooler by Zalman is an all copper cooler that has been covered in nickel for corrosion resistance, and is in the same performance category as the always hard-to-find Tuniq. Also, the Thermalright Ultra-120 is another one that is copper with a coating, so you dont have to worry as much about corrosion with all that exposed copper.
Otherwise, simply using older 'flower' style heatsinks would work fine. Since you have more space to work with, you could easily pick up multiple $10 heatsinks for older socketA/370 processors that can all be attached and have a net cooling effect many times that of a single heatpipe cooler.
Also, I would not use Arctic Silver 5 in corrosive conditions. Stick with their ceramiq... almost as good, but less to worry about.
Or, I think it would be much easier to set up some sort of thermo-electric cooling (TEC) in this case...