No need to take the Winthrop Ferry for Belle Isle, it's quite far away. Blue Line to Orient Heights and a 1 mile walk or cheap uber will get you there.
For an entry level roll, Hooks is pretty darn good. But they have limited hours, and you have to eat outside, IIRC, which might be tough in bad weather, not unlikely in April.
Any of the Luke's Lobster locations will give you an adequate roll, so will the mall "Boston Chowda" type places. Heck, I had a good lobster roll at Fenway Park once, but that was in someone else's suite. No idea what they have at the lobster roll place on "formerly known as Yawkey Way"
Summer Shack (near the Back Bay Hilton) and Legal's have pretty good options as well