I 100% agree that aquariums generally draw more of tourist crowd, I'm just not sure Maine gets enough tourists to support and any place is going to need a large enough local population to truly help sustain it. I don't think 70,000 people or so is enough. Mystic gets a ton of tourist traffic as people, but additionally has a larger community around to support it with many smaller metro areas (Providence, New Haven, Hartford) within an easy drive where they could go to Boston, but Mystic is likely closer for all of them. Portland does not necessarily have similar sized metro areas really around and I'm not sure it would necessarily have the same draw power if it is relying heavily on Boston tourists as they have a solid aquarium at home they can visit.
Reality is @CMTM has shared some great insights into this. I agree with the sentiment overall though and think Maine as whole needs a top notch aquarium before it does a zoo just because of the location of many of its bigger cities on the coast. Maine is one of the few states I have yet to visit and subsequently one of the few I have not been to a zoo or aquarium in yet.