Not to scare you or anything: I tried to get scuba certified up here in Alaska back in 2009 or so. Lost a bunch of weight and getting fit in preparation. Even went and got a full physical to make sure I was healthy enough since I'm a bit older, and I think it was a requirement with the dive shop i went to. I had a heart murmur, but the doctor thought it wasn't going to affect my diving.
Anyways first round: I got through the pool dives and was doing my final check out dives in Whittier Alaska. Also up here in Alaska, you need to get dry-suit certified as well, so I was in a full dry suit. Well, swim out to about 15 ft depth, and I start experiencing breathing issues. I'd take in a full breath of air, but I still felt like i was suffocating. So, I had to surface and was still struggling to breath. Dive instructor helped me swim back to shore and was kinda starting to freak out. Once I got back and unzipped the drysuit, I was starting to breath and feel fine, but something wasn't quite right.
Went to my doctor the following day, and my heart murmur became a heart whooshing noise. I got referred to a cardiologist and after a couple of sonograms, discovered I blew out my mitral valve. Ended up needing open heart surgery to repair the valve.
Fast forward another 6-8 months. I tried again. Doctors both my cardiologist and primary, even said I should be good to go. Did my diving in Resurrection Bay and even went to 3 different dive sites. Was feeling great too...Well, on my final dive, swam down about 30-40 feet. It happened again... I needed open heart surgery again, and this time, the surgeon replaced my valve with a mechanical one.
Sad to say, there was no third try. Doctors didn't want to explicitly say I couldn't, but I wasn't about to go through another heart surgery. The 2nd round was definitely more painful and tougher to recover from, and my heart just wasn't into it any more.