The bird with the green feet

A few days ago we were hanging out by a park with a lake.  The lake had a lots of ducks that were delighted to come out of the water and snack on some saltine crackers that another mom with a kid had brought.  As these ducks were walking around us with no fear, I noticed a non-duck, but I couldn't really tell what this bird was.  He sure swam like a duck, and had he not come out, I would have not noticed his non-duckness.  He had the weirdest feet, suited for swimming for sure with the skin flaps in between his bird toes.  But this bird had green feet, and I tried to capture these strange green feet of his/hers because they were almost fluorescent green.
What is he/her?  Do you know?

This is the closest I could get to this bird without scaring him, so I know it's not the best photo here, but come on your bird watchers out there, help me identify this sneaky bird.
Apart from bird watching, I also was trying to watch these two from falling into the lake.