A Drake (male) Hooded Merganser swimming strait towards us during my Best of Bosque workshop last December. I had committed to the 600 f/4 lens with 2X extender so I was really getting worried that it would get too close when it made eye contact - smile.
These birds are able to catch fish by direct underwater pursuit, remaining submerged for up to 2 minutes. They resurface to swallow their prey, turning it around so it is swallowed headfirst. This method avoids injury from the spiny fins of some types of fish.