The location of the containers containing these armor pieces is revealed while recovering DiMA's memories in the main quest "Best Left Forgotten". The Marine Armor is an armor set (similar to Combat Armor) which provides superior damage, energy, and radiation resistance. It provides better protection than the same armor types in its category. Some merchants in key settlements of the island also sells pieces of this armor type but expect it to be exponentially high-priced. You can get a full set by finding them in the locations marked below then upgrade them to suit your needs.
One is located inside a trunk inside a submerged container van SW of Rayburn Point.
Inside a sunken tugboat, SW of Haddock Cove.
On the deck of a sunken boat, deep under water west of the Harbor Grand Hotel.