Early GBs had a hose glassed into the keel so it could be pumped out. If you have a short hose sticking out of the bilge, mystery solved! I had one of the screws fail on the stainless strip on the stem and my keel filled up with water but only when we were underway. Drove me crazy until I discovered how it was getting in and I sealed the holes and redrilled new ones and rebedded the stem-strip. The water I pumped out was always rusty and somebody told me the keel was full of rubble, mostly scrap iron intended to act as ballast.
Many owners installed garboard drains in the keel, at the stern, to open when hauled to make sure there was no water in there.
I can't think of a good reason to go in there, but there is probably no reason why you couldn't install a plate so you could stare at the rubble. Its probably pretty smelly in there too.