We didn't really travel when our kids were that young. My son's first trip was at 12 months, but the hotel (lugano in FLL) picked us up from the airport in a vehicle equipped with a car seat. So, it wasn't an issue.
Our next trip was to Disney, where we used the Magical express. He was 18 months at the time.
My kids are now 3 and 5. We bring booster seats for them, IF we are going to do a lot of travel. IF its just a cab ride to the port, we put them in the back seat sandwiched between us, buckled in, and we hold on for dear life.
What port are you thinking of? If its bahamas, my recollection is they drive crazy down there. If memory serves, they drive on the opposite side of the street, with American cars. It looks crazy and scary as a passenger. Add to that, tourists who aren't used to being on the wrong side, and its a recipe for disaster. IMHO.
I might be inclined to skip the excursion, unless its a tour bus.