The dolphin has a crush on Kusakabe. It's jealous of Seki. How touching.
Far-fetched? Not at all. Sometimes, a dolphin in the wild or in captivity will become attached to a particular human of the opposite sex.
If the relationship continues, the dolphin may even attempt to initiate intercourse (male dolphins have been observed rubbing their erections against the thighs of women wading in shallow water).
BWAHAHAHAHAHAHA! ... Dolphin-kun has high standards for his human mistress. Seems Seki-kun has a way to go yet to "prove himself worthy" in Shinichi's eyes, even if Kusakabe did eventually accept him.
And was the "sexy time" from pages 7 to 17 a possible case of Exhibitionism (in this case, witnessed by the aquarium's *a-hem* "inhabitants")? Answer needed for ... er, scientific purposes.
Notwithstanding, another charming work by Hinasaki-sensei gets APPROVED!
I feel like the dolphin splashes him out of jealousy, perhaps...But hey, in the end, he was the one that got to make some sweet Vanilla loving with her, so I think it's pretty clear who the winner is here.
That ending though. Did he just...leave the two of them alone? So like he's maybe a little jealous but still decides to be a dolphin wingman...? What a nice dolphin.