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.