According to the U.S. Coast Guard's Office of Boating Safety, your baby should not travel on a boat — including rowboats, kayaks, motorboats, and sailboats — until he weighs at least 18 pounds and can wear a personal flotation device (PFD), also known as a life jacket or life vest, that fits snugly. Most babies will weigh 18 pounds somewhere between 4 and 11 months old. If yours hits that milestone early, consider also whether he can sit up. Being able to sit up will help your baby stay upright in his PFD and make it easier for you to hold onto him when the boat dips, weaves, and rollicks.