How much water does a person need to survive for 8 hours?

Answer is zero. Your body can survive for a day or two without water. I think the question you are looking for is how much water to take with you on a sailing trip, or how much do you need to drink each day to be healthy. The conventional wisdom on this is 64 oz ina 24 hour period (8glasses of 8oz of water). While health professionals agree that water is importaint, they do not agree that 64 oz is the "magic number" and many feel that may be too high. My question to you might be how safe is your sailing trip you may nedd to stock up on water beyond daily consumption if there is the potential for being shipwrecked and adrift without the possiblitiy of rescue.