I've had this happen to me in the past and most recently just this last weekend with a ring-necked, but with a different result. Crippled on the water, we tried finishing it off a couple times but it kept diving. Then it seemingly disappeared. It was glass calm out and we could see some air bubbles and a few small ripples, then nothing. I walked over to where it was, and only the tips of its longest tail feathers were barely sticking out of the water. I grabbed it by the leg and felt the vegetation tear as I pulled it out of the water. In deeper water that bird would've been very hard to find.
It certainly drowned, but I believe it was an attempt at escape not intentionally drowning itself.