How do you inflate a balloon underwater?

When you inflate a balloon, even on land, you pinch the mouth of the balloon shut to stop air escaping once you have run out of air. You can do the same thing underwater. Take a breath, dive under, and blow into the balloon, and pinch it when you run out of air. You could also use a snorkel, but you will need to make sure you leave enough air in your lungs to blow water out of the snorkel.