This whole issue of “struck and lost” was raised this fall in a presentation by Dr. Stenson from the Department of Fisheries and Oceans. He was dealing more with Greenland hunting and older animals. From my own experience, when an animal is shot in the water, or shot and then falls into the water, it is probably at the phase of its life when is has a huge amount of fat tissue. So most of the time the animal floats, if he died and was dumped in the water. I would be very surprised if an animal would sink in this condition, except if it was not well fed by its mother and didn't have a good reserve of fat tissue.
One of the sad stories I have on that is that this year we caught a sealing vessel hunting after the hunt ended. Because they didn't want to be caught with the seals, they just dumped them out of the boats into the water. We recovered most of the seals because they were floating on the surface of the water due to their huge amount of fat tissue.
Once again it needs to be verified and studied, but I really doubt that “struck and lost” is an issue in Canada.