Obviously it's your call, but most buses don't have seat belts to use a car seat and in the islands, few cabs do either. While I'm not totally comfortable with having my toddler on my lap in a cab, there isn't much option. But you had to do what you're comfortable with.
Make sure you read the article I wrote on this section "Taking the Baby" for helpful hints.
Luisa Frey Gaynor
Family Cruise Editor