You mentioned the difficulty with regard to post-mortem changes in cadavers retrieved from aquatic environments; I most certainly agree with you! Decedents located in aquatic environments present multiple challenges. Marine scavengers will change the presentation of any dead organic matter rather quickly, as they consider the dead human body a fresh food source and/or new real estate to occupy and colonize.
Previous research has demonstrated that complete skeletonization of a human body in off-shore Gulf of Maine locations can occur in as little as 3-4 weeks, even in winter at a depth of 600 feet, primarily due to crustacean and fish scavenging. Several species of crab and scavenging fish are common here in Delaware.
Submersion over time will drastically impact as skin and tissue decomposes. If you have ever been in a bath or hot tub for an extended period if time, even then you have probably noticed the change water has upon the skin of your fingers! In addition, there are changes due to contact with floating or submerged debris, boats passing by with propellers, tidal movement/current, conversion/change of fat into adipocere and even body re-float patterns which all alter anti-mortem appearance. ~HalAutopsy, Forensics, Hal Brown, post-mortem