i have heard that they were deepened in a later modification and that the deeper ponds with steeper bands were a liability (kids slipping in and drowning) so liability / safety concerns won out

my kids alway liked the dry basins - fun to roll down the banks, rather than dangerous
many family groups use the basin bottoms to play informal ball games = automatic gravity-assisted ball returns

I've always heard the original intent was water as well. It always sounded nice, but thinking more, the water would need to be heavily treated for algae and mosquitoes and would very likely not be healthy for human contact. I imagine it would resemble the near neon green water in Cupertino's Memorial Park...water you would not even let your dog go into.
By luck and by lawyers, we have much more usable park space instead of a green Ortho chemical pond.