Snakes are excellent swimmers, and in fact the fossil record suggests that they may have evolved as marine animals before they invaded land. Snakes therefore are not afraid of water. They have scaly skin, without hair or feathers. Therefore water won't bother them the way it bothers birds and mammals. Chickens and cats are afraid of the water because they do not like getting wet, since wet hair and feathers are bad insulation and they can get chills from getting wet. Snakes do not generate their own body heat, so they simply stay inactive when it is cold. Warm rains don't bother them. If the weather is cold, they would be hiding in safe places, either under surface objects, in rock crevices or in holes underground
There are snakes that live near but not in the water and they do tend to move away from streams and rivers when it rains because they are in danger of being swept away by flood water if they do not. Rattlesnakes do not usually live near water and in fact they are a lot more common in deserts than other snakes so they do not have to worry about being washed away by flood. OTOH, snakes like the gray-banded kingsnake and their relatives that live near streams, such as mountain kingsnakes, often can be see moving quickly away from water when it starts raining. when the weather gets hot and dry, these snakes would move back towards streams and rivers to stay cool and hydrated.