I'd never known that a search dog--or any dog, for that matter--could detect human scent through water. Not until I read So That Others May Live: Caroline Hebard & Her Search-And-Rescue Dogs by Hank Whittemore.
Caroline Hebard became interested in the unknown field of canine water search in the 1970s, in the early days of SAR. During the search for a lost hunter, Hebard's own dog, Zibo, showed intense interest at the edge of a swamp. The dog's training had taken place only on land, and Hebard had never taught Zibo or any other canine to alert on water. Ultimately, though, the hunter's body was found submerged in the swamp.
Convinced of the potential of this new aspect of K-9 SAR, Hebard began comparing notes with other handlers and, along with Marian Hardy of Dogs-East, put together actual training procedures for canine water search.
In addition to highlighting several missions involving canine water search, Chapter 14 explains the reason dogs are able to detect the scent of a submerged victim:
Scientific studies unrelated to Search & Rescue revealed that the body of a drowning victim gives off invisible skin particles with accompanying vapors, oils and gases. Being lighter than water, these rise from any depth until they break the surface and are released into the air. From the point of release, a widening scent cone forms. Detecting the cone, trained dogs will then follow it back to where the scent is most concentrated on the surface of the water, either by swimming or leaning over the side of a boat. Of course, a handler who recognizes the dog's signals will need to guide the boat operator.
Factors that can hinder a search dog's ability to detect scent through water include layers of cold water--or thermoclines--which can not only trap bodies but scents as well. Heavy algae can also trap scent, and fumes from engine-powered boats may confuse the dog. Incredibly, though, Hebard's dogs were able to detect scent even in fast-moving whitewater.
The Connecticut Canine Search and Rescue team has a nice webpage on Canine Water Search. Check it out here.