What is the difference between a split fin, a paddle fin, an adjustable strap fin and a full foot fin, and what sock should I wear with these different kinds of fins?
There are two main types of dive fins, open heel adjustable, and full foot fins, and an incredible choice of booties and socks to wear with them.  Please note, “booties”  is appropriate, “sockies” is not.  Nobody knows why that is so don’t ask.
Open heel adjustable fins are the main fins used for scuba diving. These fins are used with booties or socks, and generally have longer, stiffer blades providing more power to propel you and your gear through the water. Open heel adjustable fins, with their capability to be tightened with spring and neoprene straps, are commonly used for all types of scuba diving including recreational, commercial, military and are the fin of choice of instructors. And, because they are worn with booties, open heel fins are basically the only option for cold water diving. You’ll want to choose at least a 3mm sock for cold water diving.

Full foot fins aren’t adjustable, and they do not require boots or socks. They are generally very comfortable and divers slip into them for snorkeling, free diving, spearfishing in warmer waters. Full foot fins are ideal for recreational divers or swimmers. They’re generally lighter than open heel fins and are terrific to pack for traveling.

Once the beginning snorkeler has chosen an open heel adjustable, or full foot fin, the next question becomes a paddle or split fin model diving fin.

Paddle fins have been around forever, and are probably a good, safe choice. They are light, easily packed and provide great thrust without learning a whole new way to kick. Another great benefit is they are relatively inexpensive: You can pick up a good pair of full foot paddle snorkel fins for under $20.

Split fins are a relatively new fin technology that, when used properly, provides more propulsion with less effort. This is especially beneficial for longer snorkeling swims. However, split fins are generally more expensive, and require some instruction as to their use. Basically, instead of long kicks, you use smaller more rapid ‘flutter’ kicks. Also, split fins are better for forward motion, and less efficient for turning and paddling backward. But basically, cheap paddle fins are better than cheap split fins.