We've had a number of dogs, and all that have passed we have had to have euthanized but one. This one, my wife's chihuahua, was 14 and had gone almost completely blind. One day, my wife and son were out in the yard, looking at our new pool, when the dog got startled somehow (being almost blind, he was easily startled) and ran sideways right into the pool...the strange thing was, he didn't try to paddle or anything, he was just floating there, with his legs spread out like he was trying to float, but with his head under water. My son immediately jumped in and scooped him out, but the dog wasn't breathing...my son shook him, and the dog seemed to come back to life, as if he had been in a trance or something. He didn't seem to have breathed in any water...he didn't cough, choke, or have any difficulty resuming breathing.
A month or so later, my wife let the dog out and went to the bathroom herself...a few minutes later, she called him to come in, and when he didn't, she went out and saw him floating in the pool. She got him out, but she couldn't revive him. I think the shock caused his heart to stop, and he died from that, rather than drowning.
I got home a few minutes later, and tried CPR, but I couldn't revive him either...but I noticed that there didn't seem to be any water coming out of his nose/mouth, so I don't think he drowned. I think he went into shock and possibly had a heart attack. Otherwise, it would seem that the automatic breathing impulse would have had him inhale some water...so I don't think he was breathing after he went into the water.