Did they tell you it was being over the lake at sunset was the problem?
If so, they blatantly lied to you. Being over water could be a problem if they didn't plan their link appropriately, but sunset has absolutely nothing to do with it, other than more users on at one point in time (evening, after school/work)...
I do industrial communications for a living, and the stuff I hear about the excuses that Mesa/Skybeam/Rise gives is comical. I even use all the same gear that they do, not as WISP currently, and we never have the issues that they seem to.
Mesa/Skybeam/Rise is the WISP equivalent of Vulcan/Hesse/Blackthorne in the gun world. They suck so bad they have to keep changing their name.