People with poor vision may be able to snorkel and dive without using a prescription dive mask if they are in a shallow area such as a bay or lake and look at close things. Depending on the depth of the water, it may be possible to gain up to half a prescription strength without the use of a mask. If you are looking at something far away or you have a strong prescription (over -2), it is recommended to use a prescription dive mask to ensure a better experience.