How to make a cave?

When I import the cave SM assets and set the collision to BlockAll, I can still run through the mesh. But when import a rock and change it to BlockAll, it actually works.

Those meshes dont actually have a collision set at all, you need to use a volume for them, You can see which by loading up the island

Why does my water look like this even though everything is set up correctly

Lowering the z location of the post processing seemed to improve the visibility under water.

the ocean needs to be at a set level, check the islands and compare