Saltmarsh melons are an underwater fruit with gooey black flesh. They were also less cheap compared to water apples.
It is unknown if they are similar or related to sand melons. In Rogue Wave, at Serafina and Mahdi's Promising feast, one of the courses included saltmarsh melons stuffed with beach plums.
In Dark Tide, Becca spots a basket of marsh melons at a stall in Atlantica, and craves them. However, she did not possess enough money and had to buy water apples instead.