Snorkeling fins are used to propel yourself through water. Yes, it's possible to snorkel without them, especially in calm waters, but they make life so much easier and safer, and keep you in the water longer. Let's take a closer look:
Fins are available with closed or open foot, split or solid, and compact versions for travel. Finding fins that fit well and will not cause blisters is priority number one! They want to be snug enough to stay on, but not so tight that they hurt.
Open foot snorkel fins have adjustable straps and are meant to be worn with booties, so buy them together or get the booties first. Closed foot fins have a "pocket" that protects your heel from cuts and abrasions. They are more efficient in the water, than open foot fins, however, they also weight more. If you have the opportunity to rent fins, try both types and see which ones you like better. Remember to be aware of anything digging into your skin. If that happens, those are not the fins for you.
Paddle fins are the conventional fins that have been used for decades. Used with long, slow strokes, paddle fins help you with accelerating and maneuverability, keeping you away from sharp coral. Split fins are designed for short, rapid kicks, and there is less resistance because of the split in the middle of the fin. However, you do give up some control over paddle fins. Maybe you want to buy both types and use whichever one is appropriate for your current snorkeling adventure? Remember, though, if you choose the split type of snorkel fins, don't skimp on the price, as you will be disappointed if you do.
Travel fins are compact and shaped different than regularly sized fins. Believe me, you will be disappointed with them, so save yourself the heartache and pack your full sized fins!