I guess I had something like that in a 13' Taylor Craft built back in the early '60's. It was glass and had a plywood floor glassed in with no apparent access. This was the "air chamber" used for flotation that was used by some builders at the time. Well that may look good on paper, but in reality, boats are subject to tremendous pressures and can twist and vibrate, especially like this one that had a thin glass hull, a short deck with a lightweight plastic windshield. Not much to hold it together.
Water would enter the air chamber and periodically I would have to unscrew a little screw the builder put in the transom and let the water out.
So in saying what I just said rather than the bilge pump I had been thinking about and all, your best bet, since you don't need the pump active when on an outing, would be to install a transom drain plug, commonly used for bilge draining, between your deck and the bottom of the boat. 3M 5200 is perfect for the tube to transom interface seal. That way you can drain it whenever you wish from the outside and save a lot of effort resulting in a better solution.......you still have your air chamber. Don't know why I didn't think about that back then but I was in the service, trying to raise 4 kids, things were tight, and I just hadn't had the exposure to think of it at the time.
HTH,
Mark
