From the outside... it's hard to tell if your carbon ring is warn. (unless it's broken) The problem is... it can be warn uneven... or the boot holding pressure on it can be getting soft.
BUT... you can try one thing. Pull the carbon ring away from the stanless ring... and put a small blob of grease on the face of the ring. The grease will help seal it up. If the cavitation goes away (or is greatly reduced) then it's time for a carbon seal rebuild.
One thing... on some boats, the driveshaft has 2 or 3 clip locations. if your's has this... you can move the stainless ring up, and put more tension on the seal.
I'll post a few pics to help you check your seal for the extra clip slots.