an Apple watch?
Bah. Any £1 / $2 / €1.50 Analogue face Quartz alarm clock would take about 2 min to wire alarm contacts to suitable battery and detonator instead of buzzer. The contacts are mechanical. A detonator needs more than the 1.5V of the clock battery. An LCD digital clock alarm about x4 more. LED clocks take too much power, all the ones I know run off mains and count the 50Hz or 60Hz, though there is usually a low accuracy RC oscillator and 9V battery, but in that state the display is off and the alarm doesn't work. They start at about x5 the price of analogue face quartz alarm, or x10 there is a radio.
I can't see why you'd use an Apple watch. If you want remote operation you just use a cheap phone with anonymous SIM and a simple one transistor adaptor to fire detonator from signal on the ringing piezo speaker. Make sure you unticked the receive marketing texts / calls when buying sim or it will go off early.