Mariner was one of the crew of the Port au Prince, most of whom were massacred by the people of the Ha'apai islands, part of Tonga, under their chief Finau 'Ulukalala II, in 1806. Mariner survived and caught the eye of the chief, was adopted by him to replace a dead son, and given the name Toki 'Ukamea (Iron Axe). He remained in Tonga for four years before being rescued, being tutored by one of his adoptive father's wives.