Forensics 101: Pinpoint last verified location onboard, next, plot distance to the nearest passages that lead to a point where one could conceivably "fall overboard", do a thorough THOROUGH examination of these locations, specifically for traces of urine and fingerprints. Triangulate these locations with any/all cameras/sensors that are within sight of these locations where he could've "fallen overboard" and review all recorded images. Finally, check if he was first born child in his family. If he was, it's clearly a case of younger sibling hate/rage/jealousy.
That last sentence was an attempt at humor.