Answer and Explanation:
A hydro-thermal vent can be defined as a crevice on the floor of the sea from which water heated by geothermal energy erupts or issues. These vents are common near places that are volcanically active, areas where the movement of the tectonic plates is away from the spreading centers, hotspots, ocean basins,etc.
Marine researchers know the sites for hydro-thermal vents by computing the exact location of the place of active volcanic sites which provides the movement to the the tectonic plates which in turn produces gas and undergo some thermal processes making it easily detectable for the location of thermal vent sites.
They can be sure of finding a vent by calculating the activeness of active volcanic sites in the hydrological places.