Last year when i was in St. John my buddy and I (he's a marine biologist) ran into a group of reef squid, 3 of them. We hung around for about 45 minutes and they swam away and came back several times to check us out. We thought that was pretty amazing.
Then we ran into a group of 6 the next day, another 30 minutes of checking each other out.
Then we ran into a group of 15 the day before we left. THAT was incredible.