We were flying to the port (with infant in lap), taking a shuttle to the pier, booking excursions where they provided a van in Roatan, Belize, Costa Maya, and Cozumel. There was no need for one on the plane, we would have just paid to have it checked since he didn't have his own seat. Shuttles to the pier don't even have seatbelts, so would have just added that to our monsterous stack of luggage. Seeing as there was no guarantee that the tour vans provided would even have seatbelts and that they would be working (some are third world countries), it seemed silly to bring one at ports.
So as much as it gave me ALOT of anxiety to go an entire week without a carseat, I believe we made the right choice for our situation. Had we decided to rent a car to get to or from the pier we definitely would have had one with us, regardless of the hassle. However, in our situation, we were better off not paying the additional cost to check one on the plane ($25 each way, we can address that another day!) to simply have to lug the carseat around but never be able to use it. Choose wisely when making this decision, make sure you have considered every possible time you might need it and go from there.