Join Chef Gerry Prevost as he caters a 3-course, gourmet dinner for four aboard his own boat.    This two-hour, summer cruise around scenic Joe's Pond in Vermont is a once-in-a-lifetime experience - perfect to celebrate a special event or share the beauty of Vermont with out-of-town friends.   Chef Prevost is a professionally-trained chef who heads up St. Johnsbury Academy's Culinary Arts Programs.   In addition, he is a certified executive chef, and was the "People's Choice Award" winner at the National American Culinary Federation Convention in July 2009.