That's a good question.
Boats do flip over.
They're only about a foot wide, and there's no way to balance them without the oars. Then you either swim with the boat to shore and get back in, or you get in from the water which is tricky but can be done. Everyone who rows flips over sooner or later. When you're first learning to row you might flip over just because you don't get the oar out of the water at the end of the stroke. The shoes have velcro straps and the heels of the shoes are attached to the boat with strings so that your feet will come out if you pull very hard.