The Diving Dog

I've been vacationing the last couple of weeks at my in-laws in northern Michigan. They live on Mullet Lake which is an amazing place to be when the weather is hot and you enjoy swimming. What I especially like is how the first couple of hundred feet of lake from the shore is only 2-3 feet deep. Perfect for wading, swimming, water sports etc.... I have a black lab named Augie who also loves the water. She can swim for hours, bouncing around and getting a lot of exercise. In the last week she has started to exhibit a behavior that I have never encountered in a dog before: she dives. Augie will feel along the lake bed with her back paws for large stones. When she finds a likely candidate she flips over and dives to search the bed with her mouth. All you can see is her tail occasionally break the surface. She's been down there for stretches as long as 15 seconds. It's absolutely hilarious and somewhat bizarre. If anyone has a story of a dog they know diving, please pass it on. I'd like to know if this is an isolated occurence or common behavior.