Poisonous snakes will swim with their heads above the water because a poisonous snake swims with their lungs inflated.
So if you see a snake swimming on top of the water then it's most likely a poisonous snake.
Non poisonous snakes will be seen swimming under the water.
Although there is a snake called the coral snake that is very poisonous and sometimes swims both on top of the water and under the water.