The coast of Maine is predominantly rocky. There is a lot of granite along the coast, and the bottom can be a mix of rocks, sand, and mud. You'll need a deep-V hull, at least 18 feet long with fairly high sides. A flat bottom boat will not be safe. If you have never been up to Maine, you'll need to plan on a lot of preseason scouting to get a sense of where the birds are feeding. Look for ledges, bars, and shallow areas where birds are actively feeding offshore. As for decoys, plan on making some sleds - at least two strings worth - I would say 6 to 9 birds/sleds per string. Sleds are the best "attention grabbers" and work well at bringing birds in from long distances. You can mix in full bodied eider or scoter decoys. I think 18 to 24 full-bodied decoys would be good. Kittery Trading Post up in Maine just had a sale on their full-sized eider decoys.
As for locations, I have no idea what to tell you. I'll be doing some scouting this year as well. I have heard that the Casco Bay area is good for eider. We'll see!
Nate