I have a 93 750 SS and I have the same little black pull knob. I think it has something to do with the flush but I haven't found anything that it actually does. My flush fitting is right next to that black knob.
Ha ha ha, that is for when you sank your ski and you found a way to get it back on shore, then you pull that black knob, and all they water in the engine drains out! After that you pull your spark plugs hit the starter buttom untill there is nothing spaying out anymore, spark plugs back in and your ready to go!