apparently the sting barb pierced his heart.
The venom in the sting got him.
Stingrays have flat bodies with long, slender tails that have serrated spines. Their serrated spines can cause lacerations (cuts) and puncture wounds. The spines also contain venom. Stingrays are widely distributed in tropical to temperate waters. They are not aggressive, so an injury from a stingray usually occurs when a swimmer or diver accidentally steps on one. Stingray stings are one of the most common dive- and beach-related injuries.
Symptoms such as nausea, vomiting, fever, chills, muscle cramps, tremors, paralysis, fainting, seizures, elevated heart rate, and decreased blood pressure may develop. Death may even occur.