There are two main purposes of wearing gloves: warmth and protection.
Wearing gloves will provide extra warmth while enjoying snorkeling or scuba diving. The glove should fit snug, but comfortable, and allow as little water to enter the gloves as possible.
Gloves will also provide protection for your skin while snorkeling or scuba diving. If you accidentally scrape or bump into the coral, sea urchins, or other sharp items, the gloves will protect your fingers and hands from scrapes and abrasions. You'll also be protected from stings from things like jellyfish, sea lice, and other stinging creatures.
Gloves are usually 5-finger or lobster claw style. The 5-finger gloves are just as they sound and fit the same way as everyday gloves. The lobster claw mitts have separate finger compartments for your thumb and point finger, and a large pocket for your other 3 fingers. They tend to keep your fingers warmer, but you do lose some dexterity having three fingers in the same compartment.
Gloves made of regular neoprene tend to be less comfortable than a high quality, stretch neoprene glove. Keep in mind you'll still need to operate your dive gear wearing the gloves: use your power inflator, purge your regulator, clip and unclip buckles and straps, dig in your pocket for a dive light, grab your dive knife, etc. You need to find a balance between comfort, dexterity, and price.
A thinner, tropical glove will be made from lightweight neoprene, lycra, or other materials and their main purpose is protection rather than warmth.
Dive gloves range from thin, tropical lycra to thick, 7mm neoprene. For Canadian, cold water diving, you should wear at least 5mm thickness and up to 7mm thickness.
Care & Maintenance
Daily cleaning care will help prevent most of the funky smells to start with. At the end of each day rinse your gloves inside and out with fresh water. Hang your gloves inside out to dry the inside first, then turn to dry the outside thoroughly. Hang the gloves on an accessory hanger.
After a dive trip or a few days of diving, you may find the gloves start to take on a funky odor. Use wetsuit shampoo to soak your neoprene gear and clean the salt, chlorine, and smell from the gloves.