Hi Jim,
A few additions to Dr. Reinertson's humorous but sound advice, allow me to offer the following.
A small percentage of Viagra users complain of heartburn, a problem that can be exacerbated by the prone and sometimes head down positions assumed while diving. If you are prone to acid indigestion or gastroesophageal reflux and use Viagra in close temporal proximity to scuba, you may experience discomfort. If this proves to be a problem, your physician can assist you in control.
Also, Viagra can cause very dangerous alterations in blood pressure in those taking nitroglycerin for angina pectoris(chest pain due to coronary heart disease).
This is educational only and does not constitute or imply a doctor-patient relationship. It is not medical advice to you or any other individual, and should not be construed as such.
Best regards.
DocVikingo