Under water, sonar could pick them up, but sonar is mostly "look down". If a current dragged them somewhere outside the immediate vicinity, they'll be hard to f

Under water, sonar could pick them up, but sonar is mostly "look down". If a current dragged them somewhere outside the immediate vicinity, they'll be hard to find, because the search vessel needs to be within at most 4km to find them in the water if they are drifting. Far less on the ground, because eliminating and processing ground echos is extremely hard. If they are on the surface, getting them by sonar is not possible, but aerial surveillance might find them. However, things are very hard to see in the ocean if there are waves. They might have an AIS locator which would transmit their position and an emergency signal, but maybe they skimped on that too, because the submarine-rated ones are expensive.