What causes swollen lip after snorkeling?

A couple things: If this is a snorkel that is new to you, it may be sensitivity to a component of the mouthpiece. Some older/cheaper snorkels may have latex rubber mouthpieces, to which you could be sensitive. You may also have been exposed to a piece of jellyfish or man-o'-war tentacle. This would cause a stinging or burning sensation as well. Mild swelling can also occur from exposure of sensitive tissue to sun.