The Maine Warden Service is warning outdoor enthusiasts to watch out for thin ice.  The warning comes after a Massachusetts snowmobiler broke through ice Saturday evening on South Twin Lake near Millinocket.  Authorities say Dennis Belmonte, 43, was riding with three other snowmobilers when they lost their way on a trail and ended up on thin ice.  Belmonte was the only one to break through.  He managed to make it to safety, but his rented sled is now under 30 feet of water.  Maine wardens say local residents are generally familiar with areas where the ice tends to stay thin, but visitors are not.  They're warning recreationists to stay away from ice over any moving water such as inlets, outlets and culverts, and to avoid assuming that a trail is safe simply because it's there.