The Tech Report has compared Corsair's Nautilus 500 and Zalman's Reserator 1 Plus.
Water cooling has a lot of potential, but there are a few liabilities to keep in mind. Conductivity is probably the most obvious issue, but thankfully it's one that's easily avoided. You see, pure water isn't actually all that conductive. Tap water is, but only because of the minerals and other elements floating around in it. So, instead of using conductive household tap water in a water cooling system, most manufacturers recommend the use of distilled water..