As far as lobster in the Portland area I'd recommend "The Lobster Shack at Two Lights" in Cape Elizabeth. Food is good and the views are fantastic (they are right on the ocean). Take a look at their website and you'll see what I mean. They're about a 25 minute car ride from Portland so not sure if that would work for you but it looks like you're docked there most of the day so doable time wise but maybe a little pricey to get there and back, would have to look into that.
There are plenty of places in the Old Port area near the cruise terminal to eat lobster but I don't have any experience with them other than "DiMillo's on the Water" and I'd not recommend anyone to go there. I won't go into details but after 2 bad experiences there a few years ago I swore never to go back.
Hope that helps a little!