The Thunderbolt water taxi departs from Corozal daily at 7.00 am, and departs from San Pedro daily at 3:00 pm. However, try to confirm if the boat is operating. I've observed that on some Sundays, it hasn't made the trip, and departures have been cancelled in the low tourist season.
Tickets are purchased at the pier before departure.
Cost: BZ$45 one way; BZ$85 return.
Duration: 2 hours.