Crispin Freeman 'blew up like a balloon' when he was stung for more than 40 minutes after he crash-landed in the middle of a 3km (two-mile) swarm.
Strong currents eventually swept the 42-year-old clear of the fish and he was taken to a clinic for an emergency antihistamine injection which saved his life.
'I thought I was going to die,' said Mr Freeman, who spends half the year in the central American country with his family running a property business.
He added: 'I panicked initially but after a while I couldn't feel anything at all.
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