Stopping Apple Watch Workout when wet
First, pause the workout by pressing the crown and side button at the same time (here's an example taken from a swimming workout).



This will pause the workout until you can dry your fingers.  An unfortunate side-effect of pressing these buttons will be that a screenshot will be taken if you also turned on the feature to take a screenshot when these buttons are pressed.

After your fingers are dry, you can end the workout in the normal manner by swiping to the right and tapping End.