That aint no turtle thats a tortoise! Dont put it in water what ever you do.

Try and keep it some place warm. Where are you located that you would get them , unless someone's escaped.

I would call a wildlife place. Maybe your spca can give you the number for one. If it isnt kept warm it can die, unless you live in a hot place. You might want to mention that you think it might be someones pet.