YA Fantasy. Boy and girl in rowboat looking over side at sunken house/ city/ castle. Read early 1980s/late 1970s.

[Peter]

I read this in the early 80s or very late 70s. I remember a boy and girl in a rowboat looking over the side into the water and seeing the walls of a sunken house or castle or town. The boy dives over the side and the girl watches him swiming through the structure but when he comes up, he says that there's nothing down there, no walls, etc. My memory is pretty fuzzy but either one of them goes again, but this time when they come up, they are in an other world.

[Rainbowheart]

Long shot, William and Mary?

[SBC]

Is there anything likely on the Portal is in Water list? https://www.goodreads.com/list/show/1...

[bookel]

Do you recall the cover illustration?