Because of the fatiguing effects of the sun, wind and motion of the boat, having one drink on the water is like having how many on shore

One drink in a boat on water is like having 3 on shore. This is because alcohol hinders a person's ability to maintain posture and composure by impairing brain function. Less of a person's abilities are needed to maintain themselves on land, which is not the case in a boat, as the platform under one's feet is not stationary, and other factors such as the sun as wind are also involved.