I want to create a cave filled with water and I want my character to swim in it.
What is the quickest free route you imagine?
EDIT: This is what I want to create. It should be (semi-)automatic. Important would be that I could define holes/tunnels of a minimum size so the player fits through and halls of a maximum size.
Example:
The quick brown fox jumps over the lazy dog
There are some voxel terrain systems in the assets store which support free-form terrain (caves, overhangs, tunnels).
In the past, there have been tricks in Unity’s height map terrain to make certain quads invisible (I think the stencil buffer was mentioned), allowing you to place a model of a cave on there. This is tricky to get working right with navigation and physics, though.
A cheap option is to create a 3D model of part of your terrain with a cave in it (since a heightmap is a regular grid on X and Z, it’s easy to figure out the coordinates), place it upon the real terrain an then lower the terrain below the deepest level of the terrain under the model. If you leave 1 quad only slightly lower than the 3D model, you can avoid lighting / shadow artifacts.
Generally, I think most games would have a “cave entrance” model with a zone trigger that takes you to a “cave” scene. The cave scene would likewise have a zone trigger back to the main scene.