@alexf24 Thanks so much for posting this. I saw a small duck-like bird diving deep and swimming with big ducks here in Korea, but I couldn't see closely enough to guess what it could be. But based on your photo, I checked out info on other grebes local to us, and I think I found it!
@mike_edwards There are several types of Grebes. This is the smallest one and the only one we have locally. They are fun to watch.
@alexf24 They are! Pretty shy here, but really cool little birds that spend a surprising amount of time underwater. I think the ones I've seen here are "little grebe" (Tachybaptus ruficollis)
@mike_edwards Those we don't have. They are cute looking and they do spend time underwater, longer than you'd think.