only if you're talking about a couple/few cards on your main rig.  with many cards/rigs air is the way to go.  the best thing is to find a way to exhaust the heat to the outside.  the better and faster one can do that the cooler the cards will run while using lower fan speeds.  as for hashrate most people underpower their cards while still overclocking to get the most efficient hashrates, so watercooling won't really help unless you using them in a confined space like a main rig where they will get very hot and are running 100% power going for maximum hash.  on 1080ti's that would though be particularly helpful in that case yes since they run best hash at high power. lower model cards don't really need 100% though.