Here is what you should look for in a hockey mouth piece:
If your mouth guard is uncomfortable, you're less likely to wear it. The best hockey mouthguards, or any gear for that matter, shouldn't get in the way of your game. You need to be able to properly breathe and communicate to other players.
Custom fit mouthguards offer the best protection mouthguards can offer. Because they are custom fitted to your teeth only, they protect all the crevices that boil-and-bite guards can't get to. This also makes them more comfortable, They snap into place like a retainer and you forget about it until the end of the game. Before SISU, which creates a custom fits using vacuum suction, you could only get a custom fit mouthguard by getting impressions from the dentist or mail-order company. Whichever route you choose, playing with a custom fit mouth guard is a game changer and will improve your performance.
Most mouthguards come with a dental warranty. This means that they will cover the out of pocket dental costs in the event of an injury. Make sure to read the fine print so you know what is covered or if you need to register your gum shield. Most warranties cover you for one full year from purchase, so you should replace it every year. It's basically dental insurance for cheap!
USA Hockey:
“All players, including goalkeepers, in the Girls/Women in the 12 & under through 19 & under age qualifications are required to wear a “colored (non-clear) internal mouthpiece that covers all the remaining teeth of one jaw, customarily the upper.” Also, noting “It is strongly recommended, in all classifications, that all players wear a mouthpiece form-fitted by a dentist.”
Hockey Canada:
“For divisions of hockey that allow the wearing of the half-visor, the wearing of a mouth guard is compulsory (recommended but optional for Senior hockey).”