Kids are out in a boat in an aqueduct that was created by flooding a town. In the water below them, they see people living in their homes and are not realizing they are on the bottom of this reservoir.

[Poppy]

Kids are out in a boat in an aqueduct that was created by flooding a town.
In the water below them, they see people living in their homes and are not realizing they are on the bottom of this reservoir.
Kids try to find out what is going on.
The book was read in the last 23 years as i read it since i moved to London.
Not YA, despite the kids.

I did post this last year but cannot find the darn thing to see if there was an answer despite having the date of the original post. (embarrassed)

[Capn]

Anything on the Drowning Towns list looking familiar in the meantime? :)