When adding a child actor component to a Blueprint with the Parent Class set to one of the new Water classes and then placing it in the world, the project will crash on open. This was tested with the WaterBodyCustom and WaterBodyRiver classes and both caused the project to crash, so I believe it is all WaterBody classes that cause the issue.
The Water system doesn't exist in 4.25.4 or earlier, so not marking as a Regression.
The project crashes on launch with Assertion: WaterBodyIndex == INDEX_NONE
The project opens successfully
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-114642 in the post.