I would think that hot water would have no effect on snake or spider venom since they are ectotherms that don't become active until the temperature rises. The fact that they are most active when the mercury rises would indicate that heat wouldn't have an effect although I could be wrong. Snakes have enzymes in their stomachs that digest their food and if the temperature iisn't high enough, the food will rot in their stomachs. So at least for snakes and spiders, heat might actually help the venom along (just a thought, I really have no idea whether it would or not).