Kathleen,
 
You just snorkel as usual, except that you will need a water-proof light
source. Head lights are the best choice, because you will have 2 free hands.
Last spring I bought a water-proof head light from either Target or Wall Mart
for less than $10. I used it for snorkeling one night last summer & it worked
fine. If you think you will be snorkeling for a long time or going a long
distance away from your starting point, you may want to carry a smaller spare
flash light. Also, if you are snorkeling in an area where there are no lights
on the shore, it may be wise to leave a light on on the shore to guide you
back.
 
Watch for the eyes of shrimp at night, they reflect light.
 
Have fun.
 
A.