I've been in Commercial roofing for 18 years. Whatever you decide, do not choose a 26/29 gauge exposed fastener panel. Being in the salty environment I would do a lifetime grade of dimensional shingle with plenty of ventilation and in a black color. If you really want a metal roof, choose a concealed fastener standing seam panel with a minimum seam height of 1.5". 22 gauge galvanized (or ZAM alloy which is showing a lot of promise). If you have hips or valleys I would do full sheathing with FULL COVERAGE of high-temperature ice & water shield.
