Old Orchard Beach was on the tip of my tongue when I was writing my first response but I didn't want to lead you astray. I don't remember specific details about it but I do remember having a good time there as a kid with my family.
As long as you are on the coast the humidity won't be unbearable as you should get a nice sea breeze much of the time - and I think you've got the right idea going for nothern New England for that sort of thing.
However, if you are not tied specifically to Maine I was going to suggest some places a litte farther south (Newport, Rhode Island or Gloucester/Rockport, MA for example) that would have a little warmer ocean, just as much to do and the be within the same distance to Boston.