this is weird. An adult garter snake should, for no reason...ever... drown in a water bowl... or even almost. If he truly is almost drowning in the water bowl shanley, something else has got to be wrong with him. When you picked him up, was he as limp and lifeless as a wet noodle? Did he dangle like a piece of string? Or by lifeless, do you just mean lethargic and unwilling to move around a lot but he could still move and all that?

If he was literally limp and floppy like he was dead... then I would be concerned... but if by lifeless, you just mean he's not being very active and he's not moving around a lot... he may be attempting to self brumate. I have seen some of my snakes do this and sometimes they will choose the water bowl and just lay there and not want to move.... and when you pick them up they just kind of retain their shape. Maybe he had taken a drink, but you mistook him laying motionless in the bowl as he's drowing, and unknowlingly squeezed the water he drank out of him???

I dunno.... I just CAN"T see a healthy adult garter snake drowning in a puddle.... unless it was seriously ill... Self brumating, yes... but drowning?!