Not many people keep sea turtle as pet because they can grow quite big (1 meter in length) and they need large pool to swim around.
Most people keep tortoise as pet, these are land based. Their legs are not designed to swim and throwing them to the sea will be almost instant death to them as they will sink (they dont swim), too heavy!
Green turtle which is also kept as pet are fresh water animal, not salt water. You will kill them by releasing them to the sea because they are not used to the salt water.