Our 2017 C-24 has the same apparent problem. I was curious that the hatches on the bow deck clearly have a dark colored gasket that they are seated on. The ones on top of the cabin have no such gasket and are apparently seated against the bare fiberglass.
We have been chasing a small leak for a while in the top of the cabin. We knew it was there when we washed the boat, but we assumed it was in the caulking around the larger fittings on top of the cabin. Unfortunately, after thinking we had it sealed, we sent our boat in for maintenance.
I should note here that our boat is moored in a weatherproof boathouse, but when at the shop for maintenance it remained outside for 6 weeks during March and the first half of April in a very wet Pacific Northwest Spring. Next thing we know, there is water damage and tons of smelly blue mold and black mildew on top of the plywood panels on either side of the cabin ceiling. Good times.
This sure feels like someone forgot to install those gaskets at the factory, and only because this particular boat spent a lot of time inside did it take this long to discover.
We have now caulked around the entire hatch…time will tell.