I don’t know the fens, only passed through the area a few times, so forgive me….I’m not meaning to contradict you MercSw.
My thoughts against water making a good ‘disposal site’ are just this point…..it floods, it changes…..so surely there could be more chance that a body might become exposed over a period of time.
On the other hand, with passage of time there will be less useful forensic evidence, particularly if the victim was strangled/smothered?
And if a body is found in water, as opposed to buried, it could be difficult/impossible to exclude suicide, or misadventure?
So while writing this post, I’ve realised that disposal in water could be advantageous to a murderer, as long as the body is not discovered too soon, as very fortunately happened in the case of Libby Squire.