When we train to become scuba divers, we all learn how to share air with a buddy! It can save a life! However, the same cannot be said for sharing with snorkelers, and it can have the opposite effect. It can cost them their life!
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!!

This is a difficult topic to address since there are so many things to consider. In the article, "Effects of Aspirin on Diving" by Peter Buzzacott, MPH, Ph.D and Jim Chimiak, M.D they attempted to answer this question. 
In the first part of the article, Dr. Peter Buzzacott admitted that there just have not been enough studies to tell if there are either benefits or risks to taking an aspirin a day. The only study that he mentions dealt with rats that were given high doses of ASA for two days and then subjected to a hyperbaric chamber. In this single study, the rats seemed to have benefited by having to have fewer episodes of decompression illness. That is great for the rats, but not really helpful for the rest of us!
Dr. Chimiak MD, addressed some of the the other possible risks anytime someone is taking an aspirin regime, and that is the tendency for increased bleeding. Aspirin is a known anticoagulant, and can therefore make it more difficult for blood to clot. This is of concern to divers who may experience bleeding from things such as barotrauma and severe neurologic decompression illness (DCI). 
When a person has bleeding from injuries such as might happen with a middle-ear or sinus barotrauma (squeeze), it might be more difficult to get the bleeding to stop if the person is taking anticoagulants, such as ASA. A greater concern is with a serious neurologic DCI, where bleeding or hemorrhage occurs. If this happens, then the outcome may be negatively affected by the presence of the aspirin or any other anticoagulant drugs. 
Therefore, as with all medications and diving, consult your physician first. Then, take every precaution to avoid injuries that might lead to bleeding episodes. That means, equalize often, and take your time when descending to make sure you are experiencing no pain! Do not dive if you have congestion, or anything that might put extra stress on the blood vessels. Finally, be very conservative to avoid DCI that might lead to neurologic trauma.