Not only could it happen but it did happen. I was talking to a fellow this weekend who keeps his boat at this marina which happens to be the Queen City Yacht Club on Lake Union in downtown Seattle. He said a homeless guy on drugs swam to different boats (the docks are protected with locked gates) trying to break in. He finally found a 40' Bayliner and managed to get one of the two engines started. He first went forward into the dock and than backed out into other boats. He was banging around until one of the liveaboards got his shotgun and wounded the bad guy.
There was around $500,000 in damage with 2 boats totaled, including the Bayliner. The guy who did the shooting wasn't prosecuted because he said he was protecting life not property. Of course, he'll probably be sued when the druggie gets out of the hospital.
This was reported in the Seattle Slim, er, I mean Times.
Ron