It is true that venomous snakes do swim on top of water.
Only venomous or poisonous snakes swim on top of water while non poisonous and non venomous snakes only swim underwater.
Poisonous snakes or venomous snakes swim on top of water.
And only poisonous snakes will swim with their entire bodies visible on the water.
If you see a snake that is swimming on top of the water with their entire bodies visible then it's a poisonous snake.
Non poisonous snakes only swim below water.
Venomous snakes do also swim underwater as well.
Still even poisonous and venomous snakes sometimes swim under water so it may still be a venomous snake.
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.