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In the 16 years (numerous trips per year) that I've been diving with them, a boat has never broken down while i was on it. There have been occasions when an engine wasn't working perfectly. While i was last there a few weeks ago, Amigos did have to send out their second big boat to relieve the original out at the Atolls. Divers and gear were transferred over to finish out their day on another lovely super comfy boat.
Dive boats breaking down don't make the news, but it seems to me that I hear about incident with Amigos Del Mar more than almost any other. The first one on the list below was still the talk of the town when I was last there.
Fire Sinks Amigos Del Mars Dive Boat: Undercurrent 02/2008
Ambergris Today | Stories | Fatal Dive Tank Explosion at Amigos Del Mar
The San Pedro Sun and Visitor Guide: Boat accident in San Pedro Town