Seventeen-year-old William Tompkins of Hollywood high school has-a good start toward a two-ocean navy. For more than three years he has been whittling model ships from balsa wood, and today has a navy of 52 vessels. His tools were razor blades, pliers and jack knife, his materials balsa and Strips of sheet aluminum. The models cover a wide range of types, including dreadnaughts, cruisers, destroyers, submarines and small surface craft, all carved accurately to scale. The largest vessel in the fleet is a model of the aircraft carrier Lexington.