Rockland, ME, is something of a gritty industrial town, but it is the commercial center for that part of the coast, and has been developing a strong reputation as a "foodee" destination. It does have its own attractions, too. Camden, ME, is a somewhat upscale tourist town, the main drawback being the traffic on Route 1 right through the center of town. Camden is competing with places like Boothbay Harbor and Bar Harbor, each with their own attractions. In Camden, one of those is the Camden Hills State Park. Rockport, in between Camden and Rockland, is a quiet town sort of off the main beaten path, and a home to some renowned arts and crafts schools (e.g. the Center for Furniture Craftsmanship, and the Maine Media Workshops). Tourism in Maine is strong and has been growing, and unless we see some huge spikes in gas prices, that is likely to continue. It is, of course, strongly seasonal -- for many places, July and August are when you make almost all of your money. The Camden hills do have a ski area, but it is not a destination resort type that will be a big draw in the winter (and of course will be highly weather dependent).
So I don't see any particular red flags in terms of the region. Of course you have to look at each individual property carefully. A large number of properties for sale is probably the result of two factors, one as you've already mentioned is that there are a large number in the area to begin with; the second could be that inns might tend to come up for sale in waves -- if all the innkeepers in an area are all about the same age or have all been in the business for about the same period of time (i.e. they all bought their inns in the previous wave of sales), then it makes sense that a lot of them might all come up for sale around the same time, too..