TFE-based packings seal more tightly, have vastly less friction, are easier to adjust properly, are chemically inert. Don't know if they are less expensive but Water Control valve vendors sure charge more for graphite. 
Graphite packings are superior for high temperatures over 400F at the packing box, (not likely for a seawater application) or fire-safe services (A seawater leak will not contribute fuel to a conflagration). Graphite packings stand up better to radiation greater than 10e6 rads (again not a likely consideration).
So I continue to wonder: Why would you want to use graphite packing on a seawater application? 
BTW, as I understand it the inhibitor is a percentage of zinc chips in the graphite, so it would effectively inhibit corrosion of the stuffing box bore and valve stem from galvanic corrosion until the zinc is gone.  Once the zinc is gone the galvanic corrosion between the valve stem, graphite, and corrosive electrolyte is gonna make the packing box look like an 89-cent Wal-Mart barbecue grill used once and left out in the rain. 
I don't mean to jump on you, but I have had painful experiences where somebody reads a magazine article about the marvel of graphite packing and does a plant-wide changeout to graphite.  Then a bunch of problems popped up the I, the OEM valve supplier, was supposed to fix even though my company was not consulted before our equipment was modified. 

MORE NEWS