Re: materials for murals
Acrylics would be more durable than tempera, plus more easily sealed with a removable conservation varnish. What surface are you going to paint on or is this going to be the floor of a pool?
A painting on the bottom of a pool sure does bring up all kinds of issues that you would need to deal with. I am sure it can be done in some way that it would last quite a few years. If not done right, it would probably fall apart in a matter of weeks. I would lean towards a mosaic rather than a painting.
If you definitely want a painting, my first thought would be painting it on some kind of waterproof surface that would then be attached to the bottom of the pool rather than on the pool bottom itself, then sealing it permanently under a thick layer of epoxy resin.
Another thought was to vacuum seal it inside two sheets of polycarbonate, similar to how rare coins are sealed in what they call planchettes.