If you were to be concerned about fire retardant racks I think you have to steer clear of melamine it take a bit higher temp to burn than wood but not tons higher Pure melamine is quite high but backed melamine with particle board is slightly higher than the wood its self about 300-350ºC. PVCx are in the 550-570ºC range If you were to be looking at fire retardant racks metal or PVCx. The notion of substrate is a great one, I myself have not looked at the plastic container of it the same way since.

I actually don't know what water and sand would do with an electrical fire. I just know my friend does this and it is an interesting idea. GFIs AFCI or the rest of the similar breaker products are a relatively cheap and easy defence. I would look first to replacing the breakers with one of these types before I spend any money on anything else.