The Gulf of Fonseca is the great sheltered bite of Pacific water shared by El Salvador, Honduras, and Nicaragua. From La Unión it appears as a wide blue-grey basin broken by volcanic islands, fishing boats, and distant headlands. The geography is immediately interesting: three countries meet here, but the water feels more like one maritime neighbourhood.
Boat trips from La Unión usually combine island stops, mangrove views, and a look back at Conchagua volcano. Go early in the day, when the light is clearer and the water is calmer. This is a place where a local boatman is more useful than a fixed itinerary.