It all depends upon the individual card, and the Water Block ...
Reference cards TEND to be easier for the WaterCooling companies to just make ONE BLOCK that they KNOW fits every card from every Video Card manufacturer since ALL Reference Cards are basically the same.
However, some cards are built on second revision reference PCB boards and most WC companies make Water Blocks for those also, and many Video card manufacturers release their higher end Non-Ref designed cards on these style PCB boards ... you really need to RESEARCH your particular Video Card you're interested in, and then see what manufacturers make a Water Block for it, IF they even do.
Easiest place to start - http://www.coolingconfigurator.com/ ... just enter in your Card info, and see what they offer.
Some Other W/C manufacturers;