Bumps to avoid in Florida

I hope this is all right to post? To make it a discussion I might ask the question, “has anyone ever heard of this happening?”
I was out for a lazy afternoon sail a few weeks ago. Before all the Canadians brought their cold weather down here! Anyway I was sailing about 4.5 knots kicked back catching some rays. I had two friends with me. Bradley was sitting in the cockpit port side, I was sitting on the starboard cockpit side with the tiller in hand, and Austin was standing by the starboard winch holding on to the boom. I only had the Genoa up. Two other boats were sailing past us, we waved, they waved, we smiled, they smiled, and then just as the last smile left our faces we hit something hard enough to stop us in our tracks. Austin was nearly thrown off the boat. Bradley slid and hit the front cockpit wall, and I was thrown toward the open hatch. The hold on the tiller is the only thing that kept me from going down below.

Using proper sailor terminology I said, “What the hell was that?” Austin was dragging himself back onto the boat as best he could, Bradley jumped down below expecting to see water flushing madly through what must have been a keel removing moment, and I continued to steer over something that first sent the bow straight up into the air followed with the bow heeled to starboard 45 degrees diving into the drink. Next the whole boat rolled to starboard touching the rail while Austin was holding on to the boom for dear life.
After we righted and Bradley crawled up from below to say we had no leaks, I looked behind me to see a whirlpool right aft of the rudder with what looked like a manatee flipper or some kind of gray looking fin just under the water spinning in a circle. We sat there for a moment or two before the Genoa took up the wind and darted onward. All three of us looked in amazement behind us as the roiling whirlpool got smaller and smaller “Free Willy” sailed away.

I jumped on my cell phone to call Ray. Ray scrubs all the boat bottoms in our area. I though he could get hold of Mote Marine or the marine patrol to come over to see what I ran over. He has seen almost everything possible while diving on our boats, so I also wanted to have his opinion as to what he thought it was. I was certain it wasn’t the bottom or any obstruction because the water is 11 or 12 feet in that area, and there are no obstructions anyone knows about near that position.
Mote marine came over and found two 9’+ bull sharks. They tagged both of them. One was sporting a new white stripe on his side. It appeared his radar, sonar, or whatever they use was on the blink that day. I figured that shark helped me out a bit by scraping some barnacles off the front of my keel.