I love to use tarps on my boat in the summer. They keep the boat cool by directing the breeze into the cabin through the open forward hatch, and by keeping the sun off the cabin and cockpit. They're pretty effective during a rain. The only down side to them is that they have a tendency to be noisy and they are flammable. Outside of that, they're cheap to buy and you don't need to worry about storing them wet for short periods of time as far as mildew is concerned.