If there was a pond involved, then it's definitely Ningyo Ouji as Calamansi said.
Female protagonist, josei, lover changes from tall male to cute female, meet at a pond or stream in the park when she falls in. I just reread it recently,so those details are fresh. The two characters on the cover picture are actually the same person.
I'm pretty sure I was thinking of Ningyo Ouji. But for some reason in my head the story was mixed up with a different manga (Hagoromo Mishin, if anyone is interested), which is why I couldn't remember much lol Thank you to everyone who replied!