This is the kind of question it simply isn't possible to answer on line. It depends on what your symptoms were that caused them to diagnose a concussion, what kind of workup was done, whether you are having any persistent symptoms, whether any other injuries were associated with the accident, whether you are on any medications . . . You can see the variables.
At the very least, I would recommend a week of NO symptoms -- no headache, no dizziness, no memory problems -- before undertaking low risk diving. If you are considering diving off a boat where water can be rough or you have any risk of fall or other trauma, I'd wait for two weeks of being completely asymptomatic. The biggest lethal risk of concussion is a second head impact before the first one has healed, and even minor second injuries can be fatal. In addition, concussions can affect memory and attention span, and you should be sure you are free from such problems before heading underwater.