Here’s a screenshot from the Xen chapter:
Seems like a normal cave to me with blue water and default rock textures. In fact, it’s a possibility that this is the area in the game that the Black Mesa screenshot has expanded upon. Let’s see how it looks in context with the rest of the game.
EDIT: (This is me speaking as a member of the community, not a moderator, not a developer–because I’m not a developer as TextFAMGuy1 said.)