Those dolphin (mahi-mahi) run in schools. If you keep one in the water on hook, the school will stay with it and keep feeding. I have been in that situation too.
Fished off the rigs in Gulf of Mexico then ddove in for a swim. 240 feet of water so the black tips looked really small because they were deep. So did the hammer head until they all started to surface. 25 feet is a long way to swim calmly back to the boat when you realize how fast these animals swim. :)