[NOTE: As your question does not seem related to General Travel, but rather to Basic Scuba issues, I have taken the liberty of moving it to this forum.]
I only wear one when snorkeling, but I do carry one on the boat for those magical situations like a passing pod of dolphin, whaleshark, manta, etc., where I'll need to jump in quickly for some surface action.
You might enjoy this article on the pros & cons of wearing a snorkel I wrote for the Nov/Dec '99 issue of "Rodales' Scuba Diving":
"Although major certifying agencies continue to teach use of the snorkel as part of basic recreational scuba gear, the device's necessity is openly debated by factions pro and con. As with many equipment decisions, in the final analysis use of a snorkel post-certification is a personal one. To make this choice an informed one, here are some issues you may wish to consider.
Pros: Divers expressing a preference for the snorkel cite such benefits as: (1) easing of long surface swims, especially in rough water; (2) eliminating cylinder air consumption while swimming to points of descent; (3) locating u/w markers on beach entries and exits; and (4) allowing quick surface positioning when on board and that pod of curious dolphins or squadron of playful mantas unexpectedly appears.
Cons: Divers expressing a preference to go without a snorkel argue that they: (1) cause drag which not only adversely affects streamlining but can pull the face mask allowing water to enter, or in heavy current tear the mask off altogether; (2) pose a threat of snag/entanglement, and can be an impediment to rescue attempts requiring rapid removal of the victim' s mask; (3) float into the diver's field of vision, or the photographer's shot; (4) are effortful and inefficient. This contention includes that using a regulator while doing a surface swim is much easier, and that even if this is precluded by a low or out of air situation swimming on the back gives the diver a superior field of view and places the fins in a more efficient kicking position. Also cited are a tendency to flood (with the subsequent need to clear), retain water even after clearing, and the inherent "dead air" not expelled upon exhalation and rebreathed on inhalation.
Okay, you decide to go with the snorkel. What can you do to reduce the drawbacks? You can: (1) place the snorkel in a BC pocket, strap it to your leg, or otherwise position it so as to be readily accessible reducing drag, entanglement risk and obstruction of vision; (2) purchase a collapsible or roll up model that can be conveniently tucked away; and (3) purchase a model designed to effectively reduce ease of flooding, residual water in the tube (e.g., purge vales, reservoirs) and breathing effort. Of course, gains in on area can mean losses in another, e.g., loner tubes less prone to flooding breath harder. Your own preferences should decide the optimal balance of features."
Best regards.
DocVikingo