I might not be to helpfull because NEVER in my life have I been striper fishing in Maine, BUT however the water temperature down here in Rhode Island is pretty much the same and I have been doing fine, so have countless others. They SHOULD be there
I go to Maine every year on vacation in Kennebunkport. That is where I started Striper fishing and now I fish in NJ where I live all the time for them. The water Temperature down here is anywhere from around 54 to 57 degrees. We had a really cool May this year not hitting 90 degrees even once. The air temp has been like 10 degrees cooler than normal. The stripers down here are still around and they are catching some 50 pounders on the surf. I guess they are going to be here a little longer which in turn makes them show up later there. We also have a resident population during the summer which I fish for at night. I will be up in Maine for two weeks this year a block from the beach in Kennebunkport but I will be fishing Parsons Beach and in the river behind the golf course more. Good luck!