The “Thunderbolt” water taxi departs from Corozal daily at 7.00 am, and departs from San Pedro daily at 3:00 pm. You'll have to get another water taxi from CC to Ambergris.
Try to confirm if the boat is operating because on some Sundays, it hasn’t made the trip, and some departures have been cancelled in the low tourist season.
Tickets are purchased at the pier before departure.
When you buy your Thunderbolt ticket, ask for the contact phone number so when you're in Sarteneja, you can call and arrange to be picked up. Otherwise, I'm sure that someone in Sarteneja will have the phone number. I have a number, but don't know if it's still correct. If you get the phone number, it would be helpful to post it here.