In light of that idea Wolf suggested, I've been looking at articles on bone degradation in sea water.. The general take on it, as far as I can see so far, is that exposed bones are at much more risk of serious degradation than bones buried under silt. Forensic studies demonstrate that in most marine environments a human corpse will be reduced to skeletal form in less than three weeks, although some degree of articulation may prevail for as long as 18 months. Following the loss of soft tissue, skeletal surfaces are abraded by current-driven sediments. Physical transport of the remains by currents may result in their impact with hard surfaces to cause additional breakage and dispersal. Biological activity involving boring, encrusting and scavenging further degrades bones, resulting in advanced deterioration within 12 years, even in cases of the most durable skeletal parts (Mays, 2008: 125). ...bones are unlikely to be preserved on sites characterized by solid sea bottoms (rock or hard substrata), where they are not sealed within anaerobic contexts. http://shipwreck.net/pdf/OMEPaper11-HumanRemainsfoundonVictory.pdf Basically, if the bones are protected by layers of silt and/or being trapped in a wrecked ship (affording protection from predators/bacteria) they can survive for hundreds, even thousands of years. If they're just lying on the ocean floor, they'd be lucky to last 12 years, let alone the 20 since Stephane Hirson went missing.