I've found out that these spindles are roughly 80% efficient, so clearly you need to dissipate 300W.
Lets say you have a 10L container of water at 20°C and you run it for an hour...that will rise to 46°C if the container was perfectly insulated.
I'm not at all certain about this calculation, but I'm pretty sure that that volume of water in an aluminium box would dissipate the heat fast enough.

I've got one of these pumps and it seems good:
http://cgi.ebay.co.uk/WHALE-GP1002-1...an_Accessories
Don't see why you couldn't use that with the setup I described.