Dolphins have been documented killing themselves on many occasions. Unlike humans, dolphins have to consciously take every breath. As a human, if you try to hold your breath, there's an instinctive reaction that will eventually force you to breath. Dolphins don't have this. They'll swim down and just slowly nod off as their brain loses oxygen. The man who made the series Flipper is on record saying that one of the dolphins used on the show swam into his arms, looked him in the eyes, and then stopped breathing and died there. It caused him to rethink everything he did and made making Flipper one of his biggest life regrets. He went on to make a documentary called The Cove. It's about an annual dolphin slaughter in Japan. It's really really good.
