I was serviing in the US Coast Guard, stationed at Great Duck Island, an Island 10 miles of the coast of Bar Harbor Maine. One evening another coastie and I observed a large round white light near the shoreline, some 10 miles away. At first we thought it may have been headlights of a car, then we thought it may have been a searchlight of a helicopter, but we checked with the base and none were in the area. After looking at this light for about 3 mins it moved quickly from the shoreline to a point south of the island, it made the approx 15 miles over water in about 5 seconds. It stayed about 100 feet off the water, hovering some 5 or 6 miles off our island, it then moved away, south, and out of sight in about 8 seconds. Traveling at least 15 miles in that time, as that was how far we could see from where we stood. It made no sound.