When I checked out snapshot 18w22a, I tried to spawn in a dolphin to watch how it leads me to treasure. I gave it raw cod, the dolphin ate the cod, but no particles appeared and the dolphin didn't go anywhere special, it just kinda swam around. I checked, and there were no buried treasures/shipwrecks/ruins around. The only naturally generated marine structure around was  about 100 blocks away. I looked for a naturally-spawned dolphin, and when I gave it cod, green particles appeared and the dolphin started swimming towards a shipwreck that was about 150 blocks away. So, according to what I've seen, dolphins spawned with commands/spawn eggs will not lead the player to treasures, as long as my Minecraft works properly. I hope this will be fixed soon.

Thanks in advance,

MasterOfTheCats (David Kleiner)