What must be rated maximum voltage of the boot-strap capacitor of a high-side MOSFET driver?
In normal circumstances the voltage on the boot pin will be about 12 to 15 volts higher than  the voltage on the source pin and this is due to the charge on the boot capacitor. As this cap connects between boot and source this is the voltage seen across it normally.

However, if your circuit shuts down with a load connected to the high voltage and source the cap will need to be rated at the full power rail because one side will be taken to the high power rail by the load and the other will decay to 0 volts.

The reverse voltage rating on the diode needs to be at the full power rail by the way.