I like my RO water out of my own faucet, if we are talking about purely refreshing summer libations. Hard to beat really cold water to quench the thirst. I like a dry ginger ale in the summer, too.
In terms of adult beverages, I like a decent beer of either domestic or import origin that is not overbearing in body, which has been sitting out in my cooler after a beach and/or fishing trip, so cold that it almost hurts to hold it. Reaching into shockingly cold ice water to retrieve one adds to the experience. I'm do not require or really look for, in a summer beer, something that has an overly complex character, with hints of saw grass and coastal seaweed, etc. I want it cold with a medium body at most. I don't need a lime or orange slice, but will not turn one down if it suits the beer, either.
Mixed drinks don't really quench me. I've had and made Mojitos and they're fine, but a good old Margarita on the rocks evokes fond memories of warm weather, summer music, and friends from years gone by like few other things.