I have a C&C 24 too and have enjoyed sailing it in Lake Michigan this season after it spent over 10 years as a yard derelict.
As you have probably noticed, the manual bilge pump pickup is not in the "slot" area below the removable keel floorboard but forward of the keel trunk.
The water in this area comes from any number of sources and, unless you crawl around in the boat during a downpour (and maybe not even then), finding the source(s) of leakage will be difficult.
I solved the problem by installing a small (500 GPH) Atwood automatic/manual pump which is just the right size to sit relatively flat on the bottom of the bilge forward of the the most forward keel bolt. The 3/4" outlet hose runs aft under the cabin sole and exits next to the water tank on the starboard side - high near the molded-in stripe.
While my installation was made easier since I had to remove the cabin sole due to water damage, but I think you could route the hose the same way I did. This setup removes all but about 1" or so of any water. It takes it down further but then the contents of the hose drains back when the pump shuts off.
The pump I used is here: http://www.defender.com/product.jsp?path=-1|51|299222|84462|316441&id=123388
Why only 500 gph? Because it fits the area and is really only to remove the last bit of water. I plan on installing a manually-controlled monster pump higher in the hull to be used in an emergency.
My boat has a vent in the forward part of the cabin that we leave open all the time. I think is the Beckson C-6 Vent O Mate (4" hole required) which only costs about $26. Mine has the optional (another $26.) stainless cover which is probably needed because the plastic one is too easy to break.
It works well at keeping the boat ventillated when closed but you'd want a couple of these to make it confortable for sleeping when everything else is closed up.
I haven't checked, but both of these items may be available here on Sailnet as well.