Athlon Thunderbirds were notorious for reaching insane temperatures without any form of thermal throttling -- an uncooled Thunderbird will destroy itself without cooling. The Pentium 4, on the other hand, will throttle down and often keep running if you take the heat sink off.

I guess it's a matter of how efficiently you can cool the CPU with pool water. If you're able to keep the P4 below whatever limit it has for thermal throttling then I suspect it will, indeed, be a winner. If not, a Thunderbird will happily boil away the enire pool if given a chance.