I believe the earliest Caye Caulker Water Taxi boat to Belize City is still at 8 a.m., and Thunderbolt leaves at 7 a.m. (Thunderbolt doesn't dock at the Marine Terminal). The boats take about an hour and 15 minutes or a little less.
Here's further info:
Another option is to take a water taxi or ferry from Belize City to San Pedro. The trip, mostly on fast open boats with twin outboard engine seating 20 or 25 people takes about 75 minutes. On a sunny, calm day, it’s a great introduction to Belize; on a rainy, windy day, the trip can be wet and rough.
Several fast boats ferry passengers back and forth between Belize City and San Pedro, including Thunderbolt and Triple J. The Triple J (tel. 501-223-3464), which has a covered passenger area, makes a daily run to Caye Caulker and San Pedro at 9 a.m., leaving from the Court House Wharf, returning at 3 p.m. from San Pedro at the Texaco Wharf. Thunderbolt (tel. 501-226-2217) leaves Belize City from the Texaco gas station near the Swing Bridge at 1 p.m. Monday-Saturday, and at 9 a.m. Sunday, returning daily at 7 a.m. from its dock on the lagoon side. Probably the most accessible and convenient boats are those in the Caye Caulker Water Taxi Association (tel. 501-223-1969). These leave from the Marine Terminal in Belize City at 10 N. Front Street near the Swing Bridge. The Marine Terminal (tel. 501-23-1969, e-mail firstname.lastname@example.org) has a waiting area and two small museums to visit while you wait. With a ferry ticket, entrance to both museums costs just US$1.25. Boats to San Pedro leave at 9, noon and 3, with a brief stop at Caye Caulker. All boats charge about the same. Adult fares are US$12.50 one-way to Ambergris Caye. Children 5 to 10 go at half price; under 5 free. You disembark at Sharks pier in the middle of town. Returning from San Pedro, boats to Belize City leave at 8, 9:30, 11:30 and 2:30, plus 4:30 on weekends and holidays. Fares just to Caye Caulker are US$7.50, and boats depart at 9, 10:30, noon, 1:30, 3 and 5, returning at 6:30, 7:30, 8:30, 10, noon, 3 and 5. You can make reservations, but boarding is likely to be first-come, first-served. Reportedly, the water taxi terminal will move to the new Fort Point Tourist Village sometime in 2002. A cab to downtown Belize City from the International Airport is about 20 minutes and US$17.50.
Belize books and publications by Lan Sluder:
Belize First Magazine (http://www.belizefirst.com
Fodor's Belize & Guatemala Guide
Belize First Guide to Mainland Belize
Adapter Kit: Belize (the first and only comprehensive guide to living, retiring, working and investing in Belize -- a best-seller on Amazon.com)
UpClose Central America
San Pedro Cool (coming soon)