What kind of bird can land in a tree like a black bird, but then jump out of the tree into the water and paddle around like a duck?

We drove around the lake yesterday. The birds are all flocking like crazy, getting ready for the migration. The coolest of them were the pelicans. Such majestic birds.

However, there was another type of bird that neither Rick or I recognized. At first we thought it was a black goose because of the way it flew, with it’s head all stretched out, but in a flock they didn’t form the V formation.
Then we saw them all gathering it a big, dead tree in the lake. I’ve never seen a goose in a tree before so we definitely ruled out goose. There was one that was all alone, WAY up high.
But THEN some of them jumped out of the tree and started paddling around like a duck!
Also, several of them would stand up really tall, with their wings spread.

Unfortunately I didn’t have my Canon with me. All we had was Rick’s cell phone, so we couldn’t get any closer or clearer shots.

They were black, and Rick says they had a yellow beak, but I couldn’t see that far. I did get an impression that there might be a brown band around the neck.