If you want to go snorkeling, you are not recommended to wear your regular prescription eyeglasses. If you need vision vision aids, you can choose contact lenses with your snorkeling mask. Or you can wear a mask with prescription lenses. If you will stay a long time for snorkeling, you'd better choose prescription mask that can provide you both vision and eye protections.
Now one type of snorkel mask is made with prescription for the ones with short-sighted eyes. I am afraid that the inner space of the snorkel mask may not allow you to wear the eyeglasses. If your snorkel mask has no prescription, you could wear the contact lenses. But you should not put the snorkel mask off under the water in order not to let the dirty water come into your eyes.
Absolutely not. It is unsafe to wear glasses under snorkel mask. What's more, when you do diving, there are professional waterproof eyeglasses. You can choose the prescription. Maybe there isn't the exact prescription you need, but you can select one which is close to yours. I guess you can see the things under water with them. By the way, only people with profession training can dive underwater. If you are trained, you must know the tips of diving.
There are many options available for individuals who wear glasses for snorkeling, as glasses do not fit or work properly underneath masks. You can choose to wear corrective lenses while snorkeling, or you can purchase a mask that is made with corrective lenses built into it. Choosing the right option will make your snorkeling and diving trips much more enjoyable because you will be able to clearly see everything beneath the surface of the water.