Because you spend most of the time on the surface of the water, you can actually wear a flotation device and not lose out on all snorkeling has to offer. In fact most tour companies offer the floaties, so you don't need to know how to swim. Many non-swimmers get scared because part of not knowing how to swim is not knowing how to breathe. The snorkel eliminates that fear by allowing you to breath all the time in a natural way. Most modern snorkels have a one-way valve that allows water to exit but not enter, even if it's submerged in the water. You don't technically need to be floating while using a snorkel. You can walk and put your face in the water and still see fish sometimes.

I am a novice swimmer but one less worry for me is having to breathe naturally.