In an article by Ruth and Barry Guimbellot, in the July / August edition of Dive Training magazine addressed this very issue. They presented this scenario: you are scuba diving along a reef with a non-diving friend snorkeling along on the surface. Then, the friend surfaces dives down to you and indicates they want to breathe from your tank. What do you do? What is the harm? Just a few breaths, right?
Wrong! Remember Boyle's law from your dive class? Basically, it is the "never hold your breath when diving" law. Very simply, atmospheric pressure increases as we descend. At 33ft the air volume in our lungs is 50% of what it is at the surface. If a snorkeler dives down to you, takes a breath or two from your tank, then holds their breath as they swim back to the surface, that air will expand as they go up. It will take up twice the space it did when they took it in from your tank. If they took a deep breath, they could rupture their lungs!! This could kill them!
So, when a snorkeler asks to share air, just say no!!