if your going to buy a 780 and water cool it, get an evga 780 classified and a ekwb block. the classified is made to be waterblocked, has the best voltage controller and best power design, plus you can use the classy voltage tool to unlock core voltage, pcie voltage, memory voltage, and its pwm frequency. no other 780 allows you to control so many variables. a 780 classy under water can find a minimum 24/7/365 overclock of 1.4ghz on the core and stay well under 70c. i can bench my classy at 1.2875v core and 1375mhz on air, although admidadly i hit 85c. if i had water, i could go all the way up to 1.35v on the core which would allow likely at least another 100mhz on the core. also my vrm temps are getting up there on air too, under water they wont be a worry.
although the kraken g10 is a great idea, for extreme overclocks, the vrm's on most cards isn't very well cooled at all.