as to dogs being able to smell under-water this i am not sure of. BUT the scent particles float so someone under water would still leave trace amouts floating on top of the water and this is what the K-9 smells. I have know peeps that use bloodhounds trained to trail people and they practice a lot with water because people seam to think that water will cover up your scent. IT DOESNT. One day our k-9 unit got called to assist the game and fish to help them hunt down some poachers that ran on them. they found both of them. however the second one tried to hid under water using a blow tube to breathe. the blood hound just sat right there at the bank of the river he was hiding in the person was completely submerged under water one of the officers went into the water and got the poacher out. hope this helps U2 from any further disputing.
just my .02 worth.