Newbie with mouth pain

[5ata]

One thing I teach my students is to take your snorkel out of your mouth when you descend. it will make a huge difference on your comfort level while holding your breath. It also aids in controlling your urge to breathe once you begin your ascent to the surface.

 

[Technology_Shark]

Welcome to the board.
Your problem is most likely cause because you are excited about your adventure and therfore tend to bite on your mouthpiece to hard. It will go away with time as you become acustomed to snorkeling and the excitement wanes a little. Be concious of your bite, both position and pressure. If you find that the problem is an awkward bite position, go to your local dive store and see if you can't adapt a comfort fit mouth piece to your snorkel or possibly purchase a better fitting one if a new mouth piece is not an option.