Do you need to teach a dog to swim?

I have a 3 year old Shih Tzu who has never been in or near a pool. I am getting a pool soon and I want to know how to get him to swim. Do I have to hold him and teach him? Or can I just pick him up and drop him in and expect him to be able to swim? I don't want him to drown and I have a fear that he'll just plummet to the bottom. It's gonna be 4 feet deep by the way.