I love taking the taxi over and then flying back!
Clear customs, grab a cab outside the airport and ask them to take you to the water taxi terminal - should run you about $20 US irrespective of how many people. Buy your tickets at the terminal (they take US or BZE cash, travellers checks, Visa, No AMEX, not sure about MC) $15 one way as of last Sept. and wait for the next taxi. Someone has posted a schedule somewhere, but not sure where.
It's a great way to ease into your vacation as you watch the mainland dissappear, feel the breeze and see the cayes approach. Have fun!!