Hatch is off!
Thank you all for your help, I took the hatch off yesterday. It all went very easily. I covered the hole with a piece of white plastic sign cut to the hatch's dimensions, screwed and duck taped on the deck. It kind of looks better than the real hatch! This should keep cats from using my boat as a playground and keep most of the potential moisture from dripping in for now.
Most of the wood around the hatch was rotten and it didn't take much effort to begin removing it, especially around the corners. It doesn't look like the damage extends too far though. I'm no wood expert but I think the core is balsa, at least on this part of the boat.
I will probably dig a little further and glass it all before drilling completely through the deck and put in bolts instead of screws. At least this is my current idea unless someone here advises otherwise.
Another thing that did strike me was that on the port side, the bottom of the side has a piece of heavy cloth that didn't seem very well glassed in with some large gaps and obviously not much resin on it (see picture). Also, there is a good void between the plastic liner and the bottom of the cabin. There is a piece of something (too dark to see) that finally unite them somewhere down there and is probably a backing structure for the handrail. The curious thing is that there is no void of the starboard side. The wood trim on the forward bulkheads (where the hatch is located) show a little asymmetry too, so I assume that this dates back from the building of the boat.
Any comment on that?