Definitely one of the most challenging aspect of sea duck hunting. I've heard that Old Squaw can dive 200 feet deep...so, I assume that they dive and swim far enough away before they pop up to take a breath. What's even more challenging in sea duck hunting, is the surface of the open water is never like "glass", and there are often no markers around in the open water when you are trying to chase down a cripple, so you're not sure exactly where they went under, or where they may pop up.
And, to make it more challenging...I rarely get a successful follow-up shot when only their head and neck are sticking up above water line.....unless they're real close. Many o' shells have been wasted trying to finish-off a cripple that way.
But, the challenge keeps me going back for more.