How Can I Start a Pool Swim?

I set up my Apple watch for a Pool Swim activity before getting in the water, while I'm still wearing my glasses. Once I take them off I can't read 'Start' on the display but that doesn't matter because it's in the middle of a nice big, green button ... until the watch gets wet! As soon as the watch gets wet, the display goes black, and when I wake it, it presents the 'Siri' watch face (which wasn't the one I was using before starting the workout).

I just tested this in the bathroom sink and, no matter how slowly I immerse the watch, once it's wet the screen goes black, and when I wake it, the workout display has gone.

What am I missing? How am I supposed to start the swim workout? Before I get wet? Surely not.

Thanks in advance,

Steve = : ^ )