My Apple Watch used to automatically pick up the fact I was rowing, but, when I went for a walk with the express purpose of gaining "exercise points," I'd manually tell it "I'm going walking" and manually stop it.
My Watch is turned off and put away, but you have to go into the exercise (or fitness) Watch app and select your exercise, then, when you're done, swipe right, stop it, swipe left, scroll down and end it. Something like that. (Sorry, I'm not going to power the thing back up to get it exactly right. You should be able to find it, though.)