I have had 2 mitral valve replacements. The first one was a pig valve and didn't quite work out, so it was replaced with a mechanical valve. I returned to diving within 3 1/2 months of my operation, which was almost 3 years ago, and have been diving ever since. I do shore diving in cold water, sometimes multiple dives on the same day, and have no problems.
So, if your cardiologist clears you and you are getting monitored on a regular basis, you should be able to dive to your heart's content (no pun intended).