Oceanis 411 Sea-cock warning

My Oceanis 411 year 2000, I regard as well maintained, however this week it could have easily sunk, so if you have a similar age Oceanis please check. I returned to my berth Sunday evening at Fambridge Yacht Haven and went through my usual procedure to pack up the boat, including turning off all the sea-cocks, which I do every time and so all the levers turn freely. When I turned off the sea-cock to the prop shaft cooling, I gently turned the lever and to my shock, the fitting sheared in half just below the ball valve and lever and water started gushing through the bottom of the boat. I put my finger in the hole and called to my wife to get the wooden bungs from under the chart table seat, I pushed the bung in without much force and it held keeping all water out. It was now 5pm Sunday evening, I called the marina office on my mobile and fotunately they were still there. Fambridge Yacht Haven were superb, the person in the office called someome else down to drive the travel hoist and 45 minutes later 'Iona of Hamble' was out the water and by lunchtime Monday Althorne Engineering had her ready to go back in.
Looking at the fitting afterwards it seemed in good condition except for a bit of green staining just below the lever. It had sheared cleanly across just below the valve, there was no problem with the skin fitting so the engineer unscrewed the piece of sea-cock fitting that was left and screwed on a new one. He advised the fitting had severe electrolysis internally although all the earth wiring was clean, tight and correct. He has now put additional earth wires direct from both engine sea-cocks direct to the anode. He said he considered this type to be cheap although it was the original fitting and has fitted a more robust one. He checked all the other valves and said they were OK
I intend now to tie bungs adjacent to all sea-cocks as do IRC racing boats.
My wife and I were quite frightened to think what if! Had we been at sea sailing and this had come away on it's own, which it could have, we probably wouldn't have noticed until water was over the floorboards and then wouldn't know where it was coming from.
Happy Boating
Paul