The mask must be tempered for pressure and include a nose enclosure. Good snorkels have a purge valve. Purchase neoprene gloves and booties for protection against the cold and possible abrasions or stings from underwater organisms. Get a wet suit or dry suit, including a removable hood. Purchase a weight belt.
Good ones have lots of neat hooks and pulleys.
Buy a tank - the most common hold 80 cubic feet and are made of aluminum. These should be brightly colored. Get a diving watch or underwater timer. Purchase a diving knife. Get an underwater writing slate.