The dredging at Portland is merely removing sediment buildup within the harbor (i.e. maintenance), whereas what's being discussed at Searport is removing virgin material from the channel approaches into the harbor to allow for bigger vessels, 2 completely different animals. If you google "Searsport dredging" you'll find plenty of articles discussing strong opposition from environmental groups. I'm not saying it can't happen, merely stating that it won't be a piece of cake.
Sort of related:
I've been on Twitter for six or seven years, and I just started seeing promoted tweets from Canadian Pacific in my timeline for the first time ever. The tweet I saw was about CP's sustainability initiatives. Looks like they're trying to soften us Mainers up.