Last night, I saw a "Lassie" rerun on TV from the early 1970's.
A young man is tooling around a harbor in a little Chris-Craft runabout. He has the famed collie dog on board. He happens upon this used cabin cruiser with a For Sale sign and a telephone number on it. Does he ask permission? No. He and Lassie climb aboard and this fellow starts poking around inside the cabin. In a cabin drawer, he finds the boat's ignition key. He fires up the boat and lets the runabout drift away. He takes the cruiser out on the open water and she runs fine...for a little while. Then the engine craps out. Then this boy goes down into the engine compartment and discovered what appears to be steam coming out of the batteries. He breathes this stuff in then gets knocked out unconscious. Lassie spots shore deceptively "nearby" then jumps in the ocean to try to make a swim for shore. Then a shark starts after Lassie. Luckily, the shark and Lassie are spotted by an approaching Harbor Patrol boat with two lawmen on board. The shark is killed with the lawman's gun and Lassie is saved. Lassie starts barking to get the lawmen's attention and they spot the disabled cabin cruiser on the water. The boy knocked out in the engine room is soon discovered and then he is revived with oxygen the patrol boat happened to have been carrying. This young man is subsequently saved as well but he realizes he is in a load of trouble for acting so young, naive and stupid.
Also, there was an apparent good reason for that cabin cruiser to have a For Sale sign posted on her superstructure.
So, always ask the seller permission to test out a used boat first, wear a life jacket, have a fire extinguisher handy, make sure adequate radio communications are on board and be sure to have the proper inspection of the vessel before committing to any deals.