Saw this story as well and immediately thought of LM. They haven't yet identified even the gender of the body...
What is interesting is Sagamore creek is extremely close to Pierce Island, but it seems to be in the opposite direction in terms of the tides/currents as far as I can tell. (And also it depends on where exactly in the "creek" they found the body).
Essentially the only way a body put into the water at Pierce Island could end up in Sagamore Creek is if the current started dragging it out to sea (but through the less obvious channel, i.e. not Piscataqua main channel), it got hung up somehow or stuck in an inlet, and then maybe the tide shifted and it pushed the body back up into the creek-- the creek is at an oblique angle relative to Pierce Island's location along the Piscataqua (which is where the main tidal current is).
Here's a map of the creek:
http://goo.gl/maps/B1IFL
The area between the island and the creek is also full of little nooks and crannies and islands with small openings between the different "bay" areas. And, if my memory serves me correctly, I think at low tide there are likely large areas that wouldn't be submerged (if you look at the satellite view you can see the channels under the water).
I would be surprised if it were indeed Lizzie, but I suppose it is certainly possible-- especially if the way in which they weighted her body made it so she would only move slowly along the sea-floor and not get swept out to sea (but still able to move around and thus why they didn't find her with the initial search). AFAIK they didn't dredge the Sagamore Creek area (or any area?...just divers?).
It will be interesting to see how this plays out...