Lobster is a thing all year round lol, you’d probably get it cheaper in march. In my opinion, the “full experience” of Maine IS the cold and slushy weather. If by the full experience you mean pretty leaves and schooners in the harbor, then no, you won’t get those in March. But you’ll get to see “real” Maine! And Portland is pretty hopping all year round. Highly recommend checking out the art museum (Dahlov Ipcar is phenomenal) and the comic shops in the old port if you do visit. Hope that helps!

E: spring usually arrives sometime between late March and late May, depending on how you see spring and how the weather is acting this year. Seeing crocuses poking through snow is pretty common.