We have cruised Maine for many years in addition to living there year around for six years. In more remote working harbors such as Vinalhaven Carvers Harbor we have used vacant lobstermen's moorings in a pinch. There are two ways to identify vacant moorings. First most of the mooring pick ups are just lobster pot floats. This will be obvious as you survey the mooring field. In a busy harbor you will invariably see some floats which are fouled and overgrown with marine life. Picking that up you will usually find a stout mooring pennant that is currently unused. Another tip is that if you enter harbor after the lobstermen's workday is finished, say 4-5 PM, an empty mooring without a dinghy tied to it is probably vacant and suitable for picking up. I would suggest staying aboard just to be safe but I've never had a problem with this approach.