This is a well known problem on 2005/2004 C-25's. The factory is well aware of it. I'm surprised they haven't told everyone about it, because it's a heck of a surprise. We found it last year, and thought (at first) the hull was leaking.
The problem is a bad job of sealing the anchor locker. Water comes in via the vent and instead of going out the same hole, goes down into the compartment under the V-berth. When enough builds up, it comes out at the hole for the water line and the equivalent hole on the other side. Since the cabin floor is curved, the best way to drain the water is to drill a small hole just above the cabin floor where the V-berth bulkhead starts. And then seal it.
While you're getting the anchor locker resealed, check the rub rail behind the head. That area has also been know to leak.
Boris