A dual ball bearing turbo would necessitate water cooling MORE SO than a thrust bearing turbo simply for the fact that it will get hotter, and do so faster, not because there is less friction (which untuitively you would think would casue less heat) but because of increased response time with the shaft and capability of spinning at a higher rpm. I have had many thrust bearing turbos, never water cooled, ran fine, i currently have a ball bearing turbo also which i have not water cooled, there are no provisions for it though, even if there were, i would be reluctant to do so, as it is not DBB, but i take it as a given the life of a thrust bearing turbo will greatly exceed the life of a ball bearing turbo, DBB and SBB turbos are usually recommended for rebuild every 10k miles or so, at least thats the rule of thumb I have come across, i know none of this is technical, but just my insight