Silver Craft almost hit by our boat, and disappears into the sky in a flash, on Lake Murray S.C.

at 34.089586,-81.289049/Lake Murray, SC.  Just at sunset.  Seen from pontoon boat.  
We were heading east by southeast.  Racing to get home before dark because the lights were out on our boat.  
It was an oval to circular object.  Silver in color.  Approximately 15 ft. X 15 ft. X maybe 6 ft thick.  Couldn't tell thickness very well as it was not at a good angle to say.  
Between shallow and deep water.  
Took off from almost right under our boat.  Saw if from only feet away.  We almost hit it.  
Went skyward from the land side of our boat towards the N. W. at approximately 75 degree angle.   
Seen for maybe a half second and flies faster than anything I've ever seen, into the sky with no noise heard above our 115 hp motor which I cut immediately as it sprang from the water.  (I had my hand on the throttle at the time of sighting). 
Water was almost flat, hardly no waves at all.  It left little, if any, water splash.  
We never heard of a "water ufo" until decades later! 
Witness indicates that the date is approximate.  PD