There's a daily water-taxi service at 9 am from Punta Gorda, Belize, to Puerto Barrios, Guatemala. The trip takes about 1½ hours and returns at 2 pm; tickets cost $18 each way. A similar service operates between Livingston and Punta Gorda on Tuesday and Friday; it's a 50-minute trip. Requena's Charter Service is a reputable Belize-based outfit. Other operators have booths on the waterfronts of all three towns: you turn up and buy a ticket on the spot.
Note that life jackets are typically not provided on boats and that the seas can be rough. Postpone your trip if the weather looks bad, and don't be shy about waiting for another boat if the one offered looks unseaworthy or overcrowded.
Requena's Charter Service. 501/722–2070 in Punta Gorda, Belize. www.belizenet.com/requena.