Image via WikipediaThe answer to the first question is easy, you don't have to be an athlete to scuba dive, as when done properly, under normal circumstances, it is actually quite a relaxing sport. You'll be given (an above water) swimming test of 100m unaided or 200m with fins and have to tread water for 10minutes. Assuming you can do this, you're fit enough to dive!
Fitness and health are two totally different things and as scuba diving statistics show, most accidents are caused by underlying health issues.
Health is also a very personal thing - you cannot make assumptions that because someone else can dive with a specific health condition that you can (or cannot).
There are a standard set of questions which most dive operators will get you to answer before going scuba diving. Not all of the below conditions exclude you from diving, but it will give you a guide for the kind of things you are looking out for.
Scuba Diving Health Questions
Could you be pregnant or are you attempting to become pregnant?
Do you regularly take prescription or non-prescription medications? (with the exception of birth control)
Are you over 45 years of age and have one or more of the following:
a) currently smoke a pipe, cigars or cigarettes?
b) have a high cholesterol level?
c) have a family history of heart attacks or strokes?
Have you ever had or do you currently have...
Asthma, or wheezing with breathing, or wheezing with exercise?
Frequent or severe attacks of hay fever or allergy?
Frequent colds, sinusitis or bronchitis?
Any form of lung disease?
Pneumothorax (collapsed lung?)
History of chest surgery?
Claustrophobia or agoraphobia (fear of closed or open spaces)?
Behavioral health problems?
Epilepsy, seizures, convulsions or do you take medications to prevent them?
Recurring migraine headaches or do you take medications to prevent them?
History of blackouts or fainting (full or partial loss of consciousness)?
Do you frequently suffer from motion sickness (seasick, carsick, etc.?)
History of diving accidents or decompression sickness?
History of recurrent back problems?
History of back surgery?
History of diabetes?
History of back, arm or leg problems following surgery, injury or fracture?
Inability to perform moderate exercise (example: walking one mile within 12 minutes)?
History or high blood pressure or do you take medication to control blood pressure?
History of any heart disease?
History of heart attacks?
Angina or heart surgery or blood vessel surgery?
History of ear or sinus surgery?
History of ear disease, hearing loss or problems with balance?
History of problems equalizing (popping) ears with airplane or mountain travel?
History of bleeding or other blood disorders?
History of any type of hernia?
History of ulcers or ulcer surgery?
History of colostomy?
History of drug or alcohol abuse?
Am I healthy enough to scuba dive?
Unfortunately, it's not something you can find out online. The only way to be sure is to have a health check up by a dive doctor, which is different from your standard doctor.