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I've experienced twice and others have commented about a pint or so of water drenching the aft berth on the water ballasted C250.  Both occasions I've noticed it are after launching, though I'm not sure this is true for all other complaints of it happening.  If it is result of the launch process, my thinking is that it might be caused by the exhausting of air from the center board trunk during the launch process. 

With the stern into the water,  water fills the tube to some level and when the bow settles in, either the air in the center board trunk is compressed or it is seeks exhaust and it may do so with enough speed and force to blow water from the tube.  Either that, or the tube top with the stern down, actually is lower than the water surface during launch. 

Whatever is happening, the fix has been reported to be as simple as stuffing some rag around the cable at the top of the tube.