Well, generally, materials in Cafu can be "two-sided", and as such, they ''should'' be visible from both sides.
We'd have to check though if materials of non-solid volumes are rendered at all (Cafu might as well cull them, I'd have to look this up in the code).
If not culled, and if they're defined as two sided, no additional change is required.
Alternatively, you can always create another explicit surface that is one-sided and can only be seen from below:
- e.g. as the bottom of a non-solid brush that the player is in when on the beach and whose bottom is aligned with the water surface,
- or as a Bezier patch that is aligned with the water surface (and like the brush, one-sided and visible from below only).
In summary, I tend to think that the desired result can be achieved with the existing means, but on the other hand, I've not yet tried to make an underwater world look right. Actually giving it a try e.g. in ''BpWxBeta'' or ''JrBaseHQ'' might yield worthwhile insights though.