Can I use Ereada Mat on a waterbed

Depends on your waterbed.

There are reflective layers in the mat to direct heat upwards, but the bottom of the mat can still become rather hot.

Usually, we suggest using a towel under the mat if you put it on a foam mattress or hardwood floor, which may be afraid of heat.

If the waterbed is OK with temperatures like 120-130 F on top, it should not be a problem.

Another point is how rigid the surface is. The mat needs some form of support. It does not mean that the base should be as hard as a board, but still better if it is not too soft.

If the waterbed keeps the shape and does not wrap you when you lay on it, it should provide enough support for the Amethyst mat as well.