Hey, Kit - is that a cooling system or a pressure washer? I hasten to add that I am joking...

But seriously, I'm inclined to think that cooling requirements are over-estimated, at least for the typical home-built machine. I would prefer to avoid 240V submersible anyway - mains and water don't mix and although clearly the pump is designed to run like this, why do it at all if not needed? My 12V low-flow pump running on 5V gives more a trickle than a flow, and that's through some metres of 4 or 5mm pipe. However, the combination of narrow pipe, low flow rate, and length mean that there is quite a lot of water cooling going on in the pipe, so the water going back round via a 10litre bucket is still fairly cool. Some few degrees above ambient, maybe 10deg if I've been working for a while, but I've never noticed it much higher than that.