Oops! I guess if we're talking about a hotel in a tropical climate that happens to have a pool on its roof then this phrase could be okay. But that's unlikely. I guess I should say either в/у. I notice it's been a stumbling block for me, knowing what to use на/в/у. Always grateful for corrections!
Cool! The visual reference is a great help! I think exceptions like на пошті and на вокзалі as explained in tips/notes are part of the confusion, but I see they are the exceptions and not the rule.
на вокзалі - just think of all the platforms and rails (that definitely require "on") :)