What elevation do you live at? With Ebstein's your right heart just doesn't work very well, so you're not getting good blood flow to your lungs, which equates to possibly poorly oxygenated blood to your left heart and out to your body.
In addition, Ebstein's patients have a huge right atrium; with any heart chamber significantly enlarged there is always the possibility of a transient arrhythmia. A right heart chamber arrhythmia is normally not fatal (I don't think so anyway) but can further degrade what limited right heart function you have.
With CHD I would assume you have a cardiologist?--I would certainly recommend you consult with your cardiologist before undertaking any strenuous activity, especially if you are coming from a low elevation area.
I am a cardiac sonographer out here in western CO; I have only seen a couple Ebstein's patients in my entire career.