The mouthpieces that are attached to your scuba diving kits can change depending on your needs. Of course, the regulator or demand valve will always work in the same way, as this is what provides you with that vital pressure to breathe underwater.
However, the mouthpiece through which you inhale can be different. You can get some that are custom-made according to your mouth shape and the way you bite.
Others are pretty basic, such as those you’d find on rental gear where the material is relatively thin and can be bitten through with lots of use.
You should carefully consider how often you will be using your mouthpiece before buying. Invest in a good quality custom piece made from sturdy material if you plan to use it often.