Some sources will tell you that mallards do not dive underwater, but the mallards at Mine Falls Park will tell you different!
Update, 11/26/2007: I now have video of mallards diving in the Mill Pond in late fall. New links below.
Generally, all species of ducks fit into one of two groups: Diving ducks and dabbling ducks. Diving ducks dive underwater to obtain their food - fish, crustaceans, or underwater plants. Dabbling ducks obtain their food at the water's surface, or no farther underwater than they can reach with their long necks. Sometimes they'll upend themselves to reach a little deeper, but that's about it.
The mallard is the quintessential dabbling duck. They eat all manner of plants and small invertebrates, but only so long as they can reach them from the surface. Or so the knowledgeable sources say.
Now, ducklings of any species will dive underwater to evade predators, and perhaps to obtain food. But a full-grown mallard?
Yes, a full-grown mallard will indeed dive underwater to obtain food. I've seen it several times, and I've even captured it on video. (Left-click to view, right-click and select "save target as" to download.)
I'm not going to claim any scientifically rigorous study here, but one mallard - on video - diving underwater to obtain food proves the main point. Adult mallards certainly can and do dive.
I'll make a few more general statements here, fully aware that this is just one person's pretty-much casual observation, and generalizations from these observations have little if any scientific merit.
I've only observed this kind of behavior in one place, Mine Falls Park in Nashua, New Hampshire. I wouldn't be a bit surprised if it happens elsewhere, of course. On the contrary, I would be very much surprised if it did not happen anywhere else.
I've only observed this behavior in late autumn and winter. This suggests a generalization that just might stand up to further research. In warmer weather, when underwater food is more accessible, there's no need to dive for it, but in cold weather, when the aquatic plants and animals only live in deeper water, and when the ducks have depleted the more accessible resources, they have to dive for what little is available.
Kind of extending on that last point and tying it back to the previous, I've only observed this behavior in ducks that are overwintering in a place which they historically abandon in the winter. In recent mild winters, mallards have been overwintering in Mine Falls Park. The pond has been freezing very late in the year, and mallards can find food in deep water as long as the pond remains open. Even when the pond freezes, flowing water can remain open, and overwintering ducks can find food there, and if they deplete the food in shallow flowing water, they can dive.
The video linked above was shot on March 3, 2007, on one of the fast-flowing distributary streams that flows from the Mill Pond back to the Nashua River in Mine Falls Park. I hope to get more video later this year, as I have seen mallards diving in the Mill Pond in November.
Update 11/26/2007: Here is a video showing two individual mallards diving, shot on 11/18/2007. Sometimes they share what one has retrieved, and sometimes each duck dives on its own. I have observed at least four individuals, both males and females, diving in the pond like this, and I suspect that all of the local mallards, perhaps as many as twelve to twenty individuals, dive at various times. I have read a few articles on the Web making mention of diving behavior in mallards as a great anomaly, but it can be observed easily in Mine Falls Park in fall before the pond freezes, and less easily in the streams after the pond freezes. I have many minutes of video of this behavior for those interested. Send me a feedback and I'll see what I can arrange for you.
If this is a little-documented fact that is already well known to the experts, so be it. It is now known to some of us non-experts. But if any experts have any questions about what I am reporting here, I'll be happy to oblige.
Epilogue: The title was inspired by Dr. Robert D. Ballard, famed underwater explorer.