victor:For boat diving the snorkel is just a nuisance. Shore diving in calm conditions it is just a nuisance. Shore diving in rough conditions it is dangerous. Why do we have them when we have a tank on our back?
Not even sure we should use them between dives, I don't know but I suspect it would affect the offgassing. The more tech minded could probably tell us.
As another poster has stated..snorkeling on the surface will not significantly affect offgassing. However, you should avoid breathhold diving on snorkel while still offgassing as this can lead to DCS problems when combined with nitrogen loading from a dive.
There have even been DCS cases from breathold diving alone, but only after multiple long breathold dives.