Before purchasing your own equipment, we recommend that you try underwater hockey a few times with club-provided gear. Then talk with a club leader before purchasing your own gear. Most dive shops do not sell the kind of equipment that we need for underwater hockey (typically very minimal and sometimes with specific requirements like a two-paned mask with tempered glass).
If you plan to order from Hydro, consider talking with other club members to see if they want to join in on your order and share in the shipping cost (Hydro is located in New Zealand).
Underwater Society of America:
The following video is a good explanation of how an official game is played. Since no one city has enough players to field multiple teams consistently, we run drills and scrimmage during local practices. Playing at a tournament is when you would typically experience an official game. As our club develops, we will look to participate in tournaments when we're able. Many tournaments will also allow you to drop in as a single player if you're interested and contact the organizers in advance.