I second dire_wolf on Salty Girl: great food and atmosphere, but wasn't at all impressed with food at B&G Oysters, particularly considering high expectations we went there with. Brought a Korean from one of the two late Korean agencies there. The place was ok but the food was a let down. My date wasn't impressed too which was a bit of a bummer.
My favorite seafood place in Boston is a very simple place, a diner really, called Belle Isle Seafood in Winthrop, a town on the other side of Logan Airport, about 20-30 minutes drive from the Downtown depending on traffic. If you're looking for a place to dress up this is not it. https://www.belleisleseafood.net/
But if you're looking for a magnetic view, and great straight forward seafood right off the boat you'd be hard pressed to find anything better.
The place is in the marina. There is dedicated pier right outside of the restaurant. The boats come in and the catch goes directly into the kitchen. All fresh and tasty.
One of the best Clam Chowders in Boston imho. Get at least a cup because its very filling. But! You got to try their baked haddock dinner! A-la-la-la-la! You'd be licking your fingers long after leaving the place. You'd get few choices with dinner - their coleslaw is amazing! Crab cake dinner is also good but I'd go with haddock. Also, fresh and super delicious shrimp boat. I was a bit disappointed with scallops though.
A side note: surprisingly good burger for those not into fishy/seafoody stuff.
Cash only but there is an ATM on the premises. Also very economical considering the quality of food you'd be getting.
Salty girl is great, imaginative, deliciously prepared food in nice semi-colonial atmosphere. Belle Isle Sea Food is as straight forward as it can get. No thrills - jeans, sweats, pajamas

whatever. Just an amazing food with a view. Much better in the summer of course when you can sit outside and marvel, especially at dusk.