Boat / Ferry, Belize
Transportation by boat is great and cheap; renting a car in Belize is quite expensive. The small electric cars on Ambergris are fun.
Convenient boat/ferry between Punta Gorda and Puerto Barrios, Guatemala - just over an hour, somewhat exposed to the elements.
In Puerto Barrios you can take a small motorboat in the morning to Punta Gorda in the south of Belize. It costs about $15 and will take about an hour.