This is Not a regression.
//UE4/Release-4.26.2 CL15973114 Binary - Reproduced
//UE4/Release-4.27.1 CL17735300 Binary - Reproduced
When adding a component to a Water Body actor it should still nativized to c++ if the blueprint nativization is set to inclusive or exclusive with the blueprint having Nativize enabled.
The blueprint actor is nativized and the project compiles successfully.
Packaging fails with an Ensure condition "MyOwnerWorld [...ActorComponent.cpp] [Line: 1278]"
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-135847 in the post.