As you know, Ebstein's anomaly is a fairly rare heart condition (incidence of 1:20,000) that consists of malformation of the tricuspid valve. In the context of scuba diving, there are some troubling potential complications, which include a decrease in exercise tolerance, heart arrhythmias (surpaventricular tachycardia), atrial septal defect (patent foramen ovale, PFO), and heart failure. Often, patients present with fainting (syncope), turning blue (cyanosis), and episodes of heart-racing (palpitations). There are mild and severe forms of Ebstein's. Fortunately, your daughter has a mild form of it.
Divers should be very wary of cardiac issues since the underwater environment with its attendant increased ambient pressures can be a very unforgiving environment. If any problems are encountered during a dive, the margin for error is very slim. Moreover, the issue with Ebstein's anomaly extends into the post-dive phase since an atrial septal defect (PFO) may increase the likelihood of decompression sickness.
Discuss your daughter's case with a dive-savvy cardiologist (Divers Alert Network can give you a referral) so that you can do a realistic risk assessment. Don't be surprised if the cardiologist is loathe to clear her for diving, particularly at a younger age. When she's old enough to make her own decisions, it might be worthwhile to reevaluate the situation. Until then, staying on the surface and snorkeling carries with it a lot less risk.
