Solo, Boston Harbor Cruise works.
What you actually get is a low-effort, seated loop of the inner harbor: you pass under the Rowes Wharf arch, slide past the downtown skyline and the working container terminals, and curve out toward the Harbor Islands while a narrator names what you're seeing. On a warm clear afternoon — especially the sunset sailing — the sea breeze and the skyline-from-the-water angle are genuinely pleasant and a touch romantic. The honest catch: Boston's harbor is workmanlike rather than scenically dramatic (you're looking at cranes and ferry traffic as much as anything), the narration oversells it, and on a cool or gray day, or if you'd rather actually land somewhere, it can feel like an expensive way to sit on a boat.