"Ayer pasamos un día hermoso en nuestro barco."
Translation:Yesterday we spent a lovely day on our boat.
"Yesterday we spent a beautiful day in our boat" should also be accepted. Reported June 2018.
I think that yes literally it can be in or on but even in English we wouldn't say that really. Because you'd be ON your boat at the beach or lake or etc, on the deck. not inside of it like in the cabin or something.
It's contextual and makes the most sense to say on.
Actually you hear it both ways (unless you are on a raft). For a dingy it is always "get in".