how many people? which island? what sort of excursion--snorkel, island tour, etc? do you need translator? I have been on a zillion cruises and I NEVER pre-purchased any excursions. I mean 2-3 cruises per year for 10 yrs
I've gone with GFs and family and we always found a person just past the piers who really wanted to take you for a tour--not a ride!
If you MUST do any one particular thing--like chopper or barrier reef specialty tour, then maybe you need to pre-book. Keep in mind you wil pay more on the ship but you have the ability to bill it to your cabin. Look at tripadvisor.com for detailed advice by port. Feel free to ask more