Whether or not nutrients are degraded by "cooking" a food is up for debate, but my preference for thawing a rat is water as hot as it comes out of the tap. By the time the rat thaws, the water will feel a little cool, then you can drain it and refill with hot tap water again and let it get up to temperature for a couple minutes. Ideally, a rat should feel maaaaaybe a little warm to you, as their normal body temperature is only slightly higher than ours. Theoretically, replicating the normal temperature of the prey SHOULD produce a better feeding response, but I can't say it hurts to go a little higher, I just wouldn't use boiling water. I'd think that rat meat could start to cook at 100 celsius, and snakes aren't really meant to digest cooked meat. I have low tolerances for hot water, so my rule of thumb is not to use water hotter than I can stand to heat F/T.