Only ran a couple (as in "2") of laps on 50% and 100% surface water each. I think on 100% with rain tyres I did a 2:31, but that was by no means close to the optimum. In my opinion it's good to know how it feels in the rain, but it doesn't really make sense to try to optimize your performance in such conditions, as they will change permanently and along with it your braking points, etc.
I think it will just be crucial which tyres you choose at the beginning of the race, because you will not be able to make up the time for a pitstop (~30 sec) during the race. I like the extra strategy it brings to racing.