Some will go out the top and some will go out the purge valve. Unfortunately, purge valves will leak if they get the slightest sand under them. If a leak occurs the diver has to bend the purge valve flap back and rinse all the sand from the area. Purge valves add to the price of the snorkel. Check to see how the snorkel is removed from the mask.
Some snorkel keepers are in the shape of a figure eight.
Sliding them up and down a dry snorkel is difficult. For those, most divers simply leave the snorkel attached to the mask. However, if you have purchased a mask box to store the mask in, it will be impossible to close it with a snorkel attached. So, if you are in the latter category, purchase a snorkel with an easy mechanism for removing it from the strap of the mask. The mouthpiece should be comfortable in the mouth.