No, the boat will remain stationary by someone blowing air into the sail from inside the boat. According to Newton's Third Law of Motion, any force applied on an object elicits an equal reaction from said object. If Manny starts blowing, he will push the air forwards and in response this air will push him backwards in equal amounts. So, as long as all the air Manny's blowing hits the sail and he remains firmly on the boat, the boat will stay stationary. A better strategy would be to just turn around and start blowing backwards, ensuring none of the blown air hits the boat, which is exactly what airboats do.