I had open heart surgery to repair a valve 4 years ago. I had a mini-sternotomy to minimize the chest scar but still have a visible scar. I always carry a letter from my cardiologist when I go on a dive trip. The letter describes the surgery and states that I am fit to dive.
Remember that it's not just the fact that you have a scar...you also must disclose the operation on your medical history form and release. I have been challenged on this on two trips, so the letter is now standard and presented with my C-card.
Having said this, make sure that you really are fit to dive safely. I'm lucky in that my cardiologist is also a diver, a dive buddy, and a fellow underwater photographer.
Stay healthy.
Stan