Many of the C Dory are kept in the water full time. But, it would be a good idea to cover the cockpit with a "slant back". The hatches in most 25's deck leak to some degree--fuel tank, and bilge is underneath.
As Jim notes the balsa is well sealed in fiberglass, but any screw hole in the bottom needs to be routed out, an epoxy plug put in, and the bolt or screw thru the epoxy--so no chance of water to the core. This holds true for 80% plus of boats which have some compost core in their decks. Unfortunately there are few if any, builders who do this proper sealing of a hole in a cored structure.
We need to know where you live, and what the conditions are at your dock. I have both a dock and a boat lift on my property. I don't always keep the boat in the water, since we also "Boater home"--that is stay in the boat, on the trailer, as we pull the boat to various destinations.
The slant back cockpit cover is on the boat all of the time, even on dry land. (Unless the full winter cover is on). My son's 25 is kept in Dana Point Harbor Calif. full time in the water, and he tells me that the bilge has no water in it. He keeps a slant back over the back, in an area with little rain.
Any boat which is kept in the water need to have an epoxy barrier coat done, then bottom paint. This will prevent osmotic blistering. Blisters are rare in C Dory's--probably because most are trailered, or have epoxy jobs done on the bottom.
