About 14 years ago, Billy Litmer took a Greyhound bus from Kentucky, where he was in college, down to Key West to go snorkeling. He wanted to stay, so he went to work for snorkeling tour companies. Five years ago, he started his own business, Honest Eco Tours, specializing in dolphin watching. Then he got the idea of building his own emission-free boat that would have the least possible impact on the fragile Keys environment. The result: Squid, a 35-foot solar/electric catamaran that can hit 15 knots, but usually travels at half that speed, leaving almost no wake or carbon footprint.…