Content:
Informant – “Dolphins are considered good luck when they swim with the ship. And it’s bad luck to kill a dolphin.”

Context:
JK – “Where does this belief come from?”

Informant – “I just think that dolphins are friendly to humans. They have a long history of…there’s stories of them chasing sharks away and swimming with humans. They are sweet creatures and really intelligent. That level of intelligence demands respect.”

JK – “Where did you hear it from?”

Informant – “I just grew up with that. My father would tell me about dolphins. And there have been a couple of times in my life where I’ve actually seen it. They’ll play in the wake of the ship. It’s really neat.”

Analysis:
There seems to be very logical reasons for this superstition. So much so, that it hardly seems superstitious. Dolphins are historically friendly/helpful creatures, so a pod following your ship is definitely a good thing. It’s hard to think of a valid reason to kill a dolphin, so it makes sense why doing so would be seen as bad luck.