High heat and coatings tend to not get along unless the coating is specifically designed for high heat exposure. Waterlox products are not designed to withstand prolonged exposure to high heat.
Hot Cookware/Pots/Pans
Do not place any hot cookware directly onto a Waterlox finished surface. Use a pot holder, trivet or other insulator to prevent any direct contact with the hot pot/pan. The prolonged heat can discolor or damage the finish. The main issue is that hot cookware retains heat and does not allow it to dissipate quickly, therefore it is not just hot, but it is hot for a while.
Boiling Liquids
Waterlox finishes generally hold up well to boiling liquids and splashes. The key is that small splashes or spatters tend to cool quickly, so the duration of the intense heat is relatively short. It is OK to use Waterlox on areas near cook tops and stoves that may see occasional splashes, spills or splatter.
Warmed Plates or Hot Coffee
In general, these should not create a problem for Waterlox finishes. If they are too hot, then you can do the same damage as a hot pan. If you can comfortably hold the item without burning yourself, it will be safe to set on a Waterlox surface.
Fireplace Mantles and Other Heat Sources
Once cured, Waterlox products are relatively inert and have minimal fire risk, so they can be used around fireplaces, stoves, etc. However, if your surface is in very close proximity to a particularly warm area (i.e. a fire), the finish may soften slightly. This is almost always insignificant and is most likely to be an issue on the underside of a mantle over a fireplace, but it is something to be aware of. The finish will recover when the heat is removed.
