One of them was completely new to me, the new birder. Swimming birds to me used to be ducks or geese, so I was confused when I didn't find this distinctively colored bird in the duck section. It turns out to be a species of rail called a Sora (Porzana carolina). It is not an uncommon bird, but it apparently is very secretive and hard to see most of the time, which explains the fuzziness of the photos. It was at the far end of the pond and I was using the highest zoom setting.
The Soras range from Canada during the summer to the northern part of South America in winter, but according to Audubon, the Central Valley is one of the few places where they are resident year-round (along with the Las Vegas area and central New Mexico). Knowing that, I'll try to get some clearer shots next time!