From the AP article. This sounds a bit more plausible than a rogue stingray.
"Bertakis was apparently trying to remove the spotted eagle ray from the boat when he was stung, police Cmdr. Mike Oh said. The ray was approximately 3 feet across and 18 to 24 inches long, Oh said. Officials have kept the dead ray in case doctors need to examine it, Oh said."
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Been there, done that, the washing machine ate the T-shirt