I've seen too many boating mishaps to count. Some of the best though:
Guy backing down the ramp with the boat already unhooked from the trailer. Stops his truck, boat rolls off trailer and slams onto the boat ramp 15 ft from the water.
My dad backs the boat in but forgets to unhook the stern straps from the trailer. Trailer is floating under the boat. Dad tries to back boat off trailer, guns the engine and starts pulling the truck into the lake.
My dad backs the boat down the ramp to the waters edge. Unhooks the boat from the trailer but forgets to tie off guide line to boat. Backs boat into water, boat floats off trailer with no one in it. Watch boat float in the boat bay for 30 minutes before someone tows it over the the dock for us. At 9 years old dad decides I'm old enough to drive the boat off the trailer from now on when we go fishing

While stationed onboard the USCGC Scioto in Keokuk IA we were underway and had our 20ft john boat tied off on the portside of the work barge while steaming up river. The BM1 decides he wants to work a buoy on the starboard side of the barge. We hook onto the bouy and start dragging it around to get it back on station. While backing down and watching the bouy the stern line on the 20ft parts, the stern goes under in 18ft of water and the engines get burried into the mud, $35000 worth of ATON gear is lost over the side. Fuel cans, seats, life jackets and all manner of gear is floating down river, and the BM1 is yelling at us because we weren't watching the boat. He was an idiot.
While at the beach at Tom Stead lake in Oklahoma watch a guy trying to bring his boat into the beach. My friend yells to the guy to watch out for the rocks. Guy yells back, "What?" Tom even louder now, "Dude there are rocks right there!!!" pointing right in front of the guys boat. Guy, "What??", Tom at the top of his lungs, "Dude you are about to hit the rocks!!!! Stop your boat!!!!" BOOM!! Bang, bang, bang. Hole in the bottom of the boat, busted prop, and cracked skeg on the lower unit. Guy, "Why didn't you guys tell me there were rocks there?"

Have plenty more.