Sorry -- really am not up on cottage rental market along southern Maine coast ... it's a day trip for us when we visit that area and I'm not sure we've ever stayed in that area overnight.
If, by southern Maine, you mean the area between the New Hampshire border and Portland, there are litterally hundreds of rental properties available -- some are free-standing homes or cottages rented directly by the owners or a management company they've hired; some are part of a complex of cottages with a common office; some are parts of resort or hotel or motel complexes..... And while some are directly on the water, others may be a block or more, up to a mile or so, from the beach and the water.
Prices will vary, but, in general, tend to be several hundreds of dollars to one to two thousand dollars for the week depending on the size of the unit and its location. Summertime is prime tourist season on the southern Maine coast and demand for anything near the water is generally high and the prices reflect that. Most of the cottage owners depend on the income from summer rentals to cover their expenses for the year.
Would suggest you try a couple of things to make it easier for folks more familiar with southern Maine cottage rentals to help you -- 1) try a Google search of :Southern Maine coastal cottages", which will give you a better idea of the kinds of things that are available, and 2) get a bit more specific about whether you want to be directly on the water, what kind of price range you might have in mind, whether or not you need to be on the ocean or would settle for a more inland waterfront on a lake.....that kind of thing. With so many, varied, kinds of accommodations to choose from, it's tough for anyone to make a recommendation if all thay know is that four people want a cottage for a week in southern Maine.... just too general to help narrow the field without more info.