A catch bag is used to hold the treasures you collect while underwater.  They are often made of strong mesh which will retain its contents yet allow for instant drainage. The most important feature on any catch bag is its strength but having one that you can open single handedly and has a self closing spring is also helpful.
Knives are used for cutting, sawing, adjusting gauges, hammering, and sometimes even protection.  A good knife is one that does not rust and has both a smooth and serrated cutting edges. Your knife should be worn on the outside of your non-dominate leg or arm to allow for easy access.

In an emergency situation underwater, bang your dive knife against your tank to make noise that will travel well underwater and hopefully attract the attention of your buddy.

A dive flag is required to be floated above all divers while they are under the surface.  They are especially helpful in warning boaters of a diver’s presences. Dive flags usually come with a float but replacement flags can be purchased separately.

Wetsuits help to preserve body heat by trapping water that has been warmed by body heat so it cannot escape and take the heat with it. A wetsuit must be fit to the user because if there is too much of a gap the water will escape and you will be cold. Wetsuits come in a variety of thicknesses and styles; one-piece vs. two, shorty vs. steamer ((half sleeve/half pant vs. full-length). What you need depends on where you plan to dive most often.

Drysuits provide passive thermal protection to the wearer while immersed in water. They protect you from neck to wrist to ankle and are most often used where water temperatures dip below 15°C (60°F) or a diver plans to be in the water for long periods of time. Drysuits are designed to prevent all water from reaching the wearer’s skin and since air is a better insulator you will be warmer.

A good mask, if fit properly to your face, should allow for maximum visibility and it should not leak. When sizing a mask you should first place it against your face and then be able to hold it there without a strap just by inhaling slightly. If the mask falls off it is the wrong size. Masks come in many shapes and sizes so if the first one does not fit you should try others until you are able to hold the mask in place with only your breath. Note it is often difficult to determine this level of fit based on size alone. For a mask not to leak you need to test it before purchasing.

Snorkels are simply a tube which allows a person to breathe through their mouth while floating near the surface of the water. Modern snorkels have a sump in the mouthpiece to allow a small volume of water to remain in the snorkel without being inhaled when the diver breathes and some have a one-way output valve in the sump, which automatically drains the sump as it fills with water. Additionally many snorkels come with splash guards to help keep waves from entering the tube and DRY snorkels have float-operated valves attached to the surface end of the tube to keep water out when the snorkeler submerges. Meaning you will never need to clear the tube of excess water.

Fins come in both full-foot or adjustable strap styles, paddle style or split. One advantage of the full-footed fin is that there is no strap to accidentally break while you are out on the water but adjustable strap fins can fit any foot which is nice if you share gear among family members. With an adjustable strap fin we always recommend you protect your heals by wearing a neoprene booty underneath.
When deciding between paddle and split styling it is most important to decide if your goal is one of relaxation or speed. If you need more force and speed or if your legs are not quite as strong as your partner’s then a split fin may be the right choice. Split fins push the water back instead of up and down which allows you to cover more distance with less leg movement.