Mechanical seals like this started in pumps. Almost no one uses a packed pump any more. For good reason, the mechanicals are PROVEN to greatly reduce maintenance.
I just replaced a mechanical seal on a sludge pump on Tuesday. It had starting leaking about a 6 months ago, but only leaked when the pump shaft turned, one minute of run time each hour. The seal was really worn, the carbon face was nearly gone, about 1/8" of wear. Yet it only leaked a little, and wouldn't have sunk your boat in any reasonable amount of time.
I don't think I'd worry any more about a catastrophic failure of a mechanical seal any more than I would worry about a packing gland failing.
Of course I own an Etap, and it can't sink that way in any event, so I may be biased!
Gary H. Lucas