Arthur Borges, as I read the French, and quite possibly incorrectly, the depth of the area where the side-scan search was focused was around 3,000 to 3,500 meters, not that the sonar was being trawled (towed) at that depth.
If a second side-scan search is unproductive -- assuming there will be one -- I would think they would need both that and other survey tools to first map the contours of the bottom to high detail. With a contour profile in hand, they could deploy the proverbial torch or flashlight (a submersible with video and lights). And even then I would say -- to continue the BEA metaphor -- they would not intend searching the whole of Switzerland, but only part of one canton.