Hi Tammy Ė
Iím sorry about your experience. And I know how you feel. Itís kind of like something out of an Edgar Allen Poe story, isn't it? Since I donít have much experience with brumating temperate climate snakes, nor have I kept many garter snakes, I really canít evaluate your particular set-up. But for future reference, I do know that it is quite common for some snakes to hibernate under water. Itís not just an amphibian or turtle thing! So weird as it looks, if you see that again and all your other conditions are okay, just trust that the animal has made a natural choice.
Iíll leave it to a veterinarian to explain, because I really cannot, but the heart of a reptile can continue to beat even though it is essentially dead. I know that sounds like a complete contradiction. Letís just say that it can be exceeding tough for even the most experienced person to determine when a snake has finally died. I have cut open more than one snake for necropsy and observed the heart still beating. I knew those animals were dead by any other definition, had no possible chance of recovery, and did not make any observable response to painful stimulus. Nevertheless, if I find a ďpetĒ snake dead now, I tend to leave it alone for an additional length of hours to give myself a cushion so I donít have to second guess myself when I cut into its body. You can find the position of the heart fairly easily on a healthy snake by turning it over and watching for the heartbeat. Youíll notice it doesnít beat very fast! When it is very close to dead (or very chilled), minutes could possibly elapse between beats.
I can remember the first (and only) time I fully opened an incubating python egg which I was positive had gone bad, and found a delicate, perfectly healthy-appearing embryo with a beating heart and no chance now of survival. But I learned a valuable lesson.
I can see youíre turning it over and over in your mind. Try not to. Youíll never know for sure what happened, but the fact that the other two became active is a good indication that something was way wrong with the one that didnít. You were doing what you thought was best.