Yes I have seen two ducks drown themselves about 8 months apart. The first time it was to far away so I didn't believe what I saw.
The second was in a quite cove with a friend of mine we were fishing, the water was very calm and dried leaves on the bank. We watched three ducks fly in the cove and land, they muddled around a bit like ducks do. One of them 60 or so feet away dove under the water which my friend and I thought he was eating the vegetation under the water like they often do.
Well he did not resurface, My friend and I sat very quite for 5 minutes or so not saying a word to each, by this time we had stopped fishing and was searching the water for any movement, a ripple, a bubble anything resembling life, it never came, we scanned the bank thinking he swam to shore and out. Nothing, the leaves on the bank were very dry we should have heard something. We thought a large bass may have grabbed his head but there should have been a fight of some kind there was nothing.
A minute or so later the other two ducks took flight and left the cove. I have never seen anything like this since that day. It has became a permanent memory in my head. Some time later I did find an article on the web that said in certain conditions a sick duck will drown itself to strengthen the flock by diving down and grabbing a branch or something stationary on the bottom.