Wait for a nice day, unless you are a fearless pro. The worst weather is windy weather. Rain is OK, but not while you're applying. Tarp it off if you have to. Vinyl should never be applied in temperatures less than 40 degrees Fahrenheit or 4 degrees Celsius. Also the lettering surface should be free of dirt and wax for best results. Done properly, this job will last for over seven years, (or until you sell the boat.)