The reason why heart transplant patients can't go in the ocean is mainly due to a risk of the heart transplant patient developing an infection from bacteria and other organisms that can be present in oceans.
When a person has a heart transplant they are also often taking immunosuppressant medications that are crucial for preventing organ rejection, including preventing rejection of the transplanted heart.
The immunosuppressant medications can and do weaken the patient's immune system, which makes the heart transplant patient more susceptible to illnesses from the germs that can be present in ocean waters.
Although you can still swim in the ocean if you prefer to do so but it's not recommended.
Heart transplant patients should swim in properly chlorinated pools or other water that is safer than ocean water.
However you should always wait until after you healed from the heart transplant and wait until the doctor or transplant team says it's okay.
Although doctors do say that a child or anyone else that has had a heart transplant can swim in the ocean or chlorinated pools after transplant.
Because the adults or child's immune system will forever be weakened after transplant, the doctors recommend that the adult or child does not swim in lakes or ponds as they are more likely to have bacteria or other organisms than oceans and pools, which could cause the adult or child to become sick.