My son wore one of the guards from Dick's Sporting Goods(that you boil) for years playing basketball without incident. However, this fall my son got hit in the mouth with a baseball and had a front tooth knocked out only 3 months after getting his braces off. Granted, this may have been just a freak hop, but he now wears a custom fitted mouth guard from the dentist. Spend the money and get a good one. It fits better than the $25 kind and they will be more apt to wear it. He is an infield player.
I will put in a plug here to make sure every team has team baseball insurance. USSSA has two levels of coverage (for all age groups) that is more than reasonable. In our experience, our medical coverage paid for the emergency room (the tooth went through his lip and he needed stiches), but replacing the tooth was not covered under either our dental or medical plans. Thank goodness the team had coverage that picked up most of the difference. Also, insurance won't cover the replaced tooth if it should get knocked out again....another reason for the guard.
Keewartson just told me that no one knows that he is wearing one when he plays. It is clear.