Scuba Diving. Lots of people have heard that your lungs can pop or something similar and it makes them really afraid to try it. If you hold your breath, you may have issues with your lungs but your SCUBA apparatus is such an amazing design that even if you need to throw up underwater it's designed to filter your vomit through the apparatus so you can continue breathing even after throwing up THROUGH it (which you should do if you feel nauseous down there). Just keep the apparatus in your mouth and don't stop breathing and you'll have a great time.
Scuba Diving is one of my favorite things to do and I really think more people should try it!