For asthmatics, the biggest concern is having an attack underwater. And some studies have shown an increased risk for arterial-gas embolism or decompression sickness, Lambert said.
For instance, one study reported by DAN found that 12 percent of arterial-gas embolism victims had a history of asthma, and another found that asthmatics have a four-fold increased risk of decompression sickness.
But Lambert said the data were based on surveys and case reports, detracting from their strength and power.
A large study at the University of Rhode Island found only one asthmatic patient among 2,131 diving death records that they assessed.
Guidelines from the American Thoracic Society say that patients with well-controlled, stable asthma who have normal spirometry and "understand the risks of scuba diving … seem to have only a slightly increased risk over the general population."
Patients considering diving "should have their asthma severity and control reviewed, undergo spirometry, and have an action plan in place with access to emergency rescue medications," according to the guidelines.
Ultimately, the decision about permitting the asthmatic patient to dive is in the physician's hands, Lambert said.
He said patients with cold-induced asthma are immediately prohibited from the sport because exposure to colder underwater temperatures could trigger an attack. Likewise, exercise-induced asthmatics are disqualified from diving.
But those with mild intermittent and mild persistent asthma are "probably OK to dive," Lambert said.
"It's the patients with moderate persistent and severe persistent asthma … that I'm going to be most concerned about," he said.
He said physicians should base their decisions about a patient's ability to dive on chest X-rays, pulmonary function tests, and a thorough patient history -- and make all decisions "on a case-by-case basis."
Most importantly, the physician must be sure the asthmatic patient understands what he or she is getting into.
"Diving with asthma is all about patient education and understanding the risks," Lambert said.
Dr. Matthew Partrick, who specializes in emergency and undersea and hyperbaric medicine at Southern Ocean County Hospital in Manahawkin, N.J., said patent foramen ovale (PFO), literally a "hole in the heart" between the atrial chambers, increases a diver's risk of decompression sickness.
Dr. Alfred Bove, of Temple University and president of the American College of Cardiology who is an expert in diving and the heart, said most patients with cardiovascular disease can participate in recreational scuba diving as they would any other sport.
Bove said deciding which patients with heart disease should or shouldn't dive is an "art," because there is little data on risk.
As with asthma and diabetes, he said, decisions are made on a case-by-case basis. But in general, he said patients are diving "with stents, pacemakers, mechanical heart valves, or while they're taking blood thinners."
"Many people who have heart problems, can dive safely," he said.