Most wild ducks have a small amount of fish in their diet, nothing to worry about. The ducks that you are seeing are probably Lesser Scaup. At a distance the only colors that you are able to see are black and white and they are divers, which would account for their extended dives.

Even if they are mergansers, which have a diet primarily made up of fish, they won't stay long enough to cause any real damage. Enjoy them while they are there. I raise Migratory waterfowl here in Texas and have little visitors, like your (probable) lesser scaup all the time. Consider it a cool visit from a distant traveler.

Thanks, Newbee