hapkiman

Why do you want to water cool it?  Are you having heat issues?  I have this exact same card, and admittedly have good case air flow, but my card idles in the 20's (C), and hits low to mid 60's (C) while gaming.  That is excellent.  I've never seen my card even hit 70C.  It is a very well cooled card.  I use MSI Afterburner and set a fan profile of 60% and it is silent.  If it's a super hot day I might even set the fans to 70% - but you really cant hear anything.  They are great fans.  If you really need to water cool it, I would sell what you have and get one of the AIO cooled versions by MSI or EVGA. Opening it up or modding it will void your warranty.

It is very easy to replace the thermal compound on anything, either a graphics card or a CPU.  Just go slow, and take your time.  Use a higher than 90% alcohol to clean off all of the old paste (Q-tips work great for this) and then apply a very small amount of the new, about the size of a BB.  Don't use too much!  Let the heat spreader smash it out as you reassemble it.  I recommend MX-4 as it is very high quality, and doesn't require a burning in period.
 
Just remember that you could void your warranty by disassembling your card.  Check first to make sure.