First of all, keep in mind that I am unable to diagnose you because I am unable to examine you, this forum is for educational purposes.
Changes in the pressure as one dives can put pressure on the dura as the condition of the various gases change. It is conceiveable that these changes at deep water pressures may affect your graft, based on the surgical techinque used and the health of the graft. Since I am not in a position to review the surgical technique used and the current condition of the graft, I will defer to the neurosurgeon who placed the graft. If at any time you develop headaches that worsen with standing and then improve on lying down, or develop a clear fluid dripping from your ear or nose your should be concerned about a leak in your graft. A MRI with CINE flow study and/or CT cisternography will help evaluate the current function of the graft.
I hope this has been helpful.