**STOP PRESS**
After my last (long winded) post I got to thinking and went out and had a good look around. I found my water leak problem...
It's the transom locker (wet locker) drains.
Inside the transom locker there are two small drain holes - one on each side. They both have very short hoses to (supposedly) drain water from the locker into the bilge. The hoses aren't that obvious because they only just stick out past the soundproofing on the underside of the engine hatch - so I've never really noticed them before.
This arrangement kinda works OK on the starboard side... but on the port side the short hose means that the water drains out right onto the raised "false floor" area I spoke about in my last post (the floor that the fresh water tank and battery sit on).
If the boat is slightly bow-down, the water from there travells forward along the "false floor", under the cockpit floor, and comes out under the cabin bulkhead below the cabin door straight onto the port side seat in the cabin. From there it can either run into the underseat locker or down onto the cabin floor.
Even if the boat is bow-up (particularly if it's listing ever so slightly to the port side) the water simply pools behind the fresh water tank. As soon as you hitch the boat up to a vehicle (and the bow drops) any water that collected there will then run forward under the cockpit floor and into the cabin. And you'll have a wet floor when you get to the boat ramp.
Of course the wet locker isn't meant to be water tight (which is why it has drains in the first place) so washing the boat, or heavy rain will cause enough water to easily enter and end up wetting the floor. This explains why I had almost a gallon of water in the cabin locker after 1 wash.
The fix... Simply connect longer hoses from the transom locker drains to let the water drain directly into the very bottom of the bilge (preferrably below the stringers). Probably only necessary on the port side but I plan to do this on both sides.
Anyway, that was my cause of my leak, and it also explains why I often had water pooling above the stringer in the bilge (which would often have to be sponged out).
I hope it helps others.
Ric