For a marina, I would recommend units that a specifically designed as outdoor units. The Engenius units are inexpensive -- http://www.newegg.com/Product/Product.aspx?Item=N82E16833168118 and designed for outdoor usage. You would use a single Cat5 to feed signal and power. You could use direct burial rated Cat5 for extra durability. A small POE switch could allow you to run several access points and supply power to all of them.
For a marina, I would recommend units that a specifically designed as outdoor units. The Engenius units are inexpensive -- http://www.newegg.com/Product/Product.aspx?Item=N82E16833168118 and designed for outdoor usage. You would use a single Cat5 to feed signal and power. You could use direct burial rated Cat5 for extra durability. A small POE switch could allow you to run several access points and supply power to all of them.