I see this behaviour in Anchorage during spring all the time. Sometimes,
its obviously associated with bathing. At other times, when some threat is
near such as an eagle flying overhead, I've seen it occur - maybe as an
attempt to make themselves a less likely target? And when other geese get
to near a mate, I've seen one male chase another male (I assume they are
males) and the one being chased becomes a submarine. I don't believe I've
ever seen it associated with feeding.

>Not sure why they wouldn't just fly off.

A lot less energy is used diving rather than flying. It takes a lot of
energy to get a goose sized body off the water.

Andy Sorensen
ajstudio at ptialaska.net