No one will be able to give a definite answer, so I may as well off a research direction that I happened to see on a This Old House segment.

There are a few special plaster mixtures and treatments used for water proof surfaces to have seamless-looking walls/floors with water fixtures like sinks, showers, etc. On of these is called Tadelakt, a type used in Moroccan architecture. The linkes article also references a few other types from elsewhere in the world.

No one will be able to give a definite answer, so I may as well provide a research direction that I happened to see on a This Old House segment.

There are a few special plaster mixtures and treatments used for water proof surfaces to have seamless-looking walls/floors with water fixtures like sinks, showers, etc. On of these is called Tadelakt, a type used in Moroccan architecture. The linkes article also references a few other types from elsewhere in the world.