Are high power ceramic resistors waterproof?
On the off chance that they get wet, will it chance their resistance?


Yes.

Unless they are sealed, yes. How much it changes depends on a few factors (impregnation depth, chemistry of the solution, volume, mass, and temperature). 

They are made from a porous ceramic powder material so they will absorb moisture from the air and liquid in surface contact. However, if you run enough power through them, they will get hot and "bake" the moisture out. The bigger problem is the impurities left behind by the liquid. 

Seal them.

For reliability, seal them with an automative-grade silicon sealant and derate their power rating by 2.

What are you using them for exactly? (just curious)