From what I understood, there's no clear explanation. But I believe it has something to do with strong attachment (love or hate) to human. For dolphin instructor guy, his former trainer throw away the whistle to the ocean after he was released there. He took it and lost his consciousness. He was in a human form when he stranded on the beach.
For pink dolphin guy, he was a wild dolphin, unlike mr instructor. He was caught by fishing net. That's why he hates human and being all rebellious.
For the orca, I have no idea why. He was stranded on the beach as well I think.
so wait in korean lezhin, this webtoon is finished now???

I read this on lezhin up to chapter 59 in korean. It's seriously good. Couldn't believe that in later chapters the dolphin guy aka hot mr instructor can be so sweet to MC. At first, he wasn't really care about MC, he just want to have sex and eat sashimi.
Spoiler ahead. Don't read if you hate spoilers.
1) Dolphin instructor guy is looking for his trainer when he was raised in an aquarium. They separated after he was released to the ocean, and the whistle actually belonged to his former trainer.
2) His trainer turns out to be MC's former colleague/boss. So, MC feels left out when they reunited.
3) After the story between dolphin guy and MC ended, the author focus on mr trainer, who "raise" a male orca in his room.