carmskarma,

No, the sea definition is stored in the neighbourhood mod I'm fixing now.
But, I'll update the terrain water later, too as well as releasing the other variants of the series. I'm combining some of the different versions.

Have you tried to build a swim-pool in a beach lot to see if anything happens, too?
The "new" beach is actually the NL lotskirt sea as well. It's been refined with the pool PS2 surface, a thing I would do later.

Also, may try to remove the swim-pool in the beach lot but keep a pond and this mod to see if it's the pool tool conflicts with the beach code-structure and results in the same thing you saw.