A leak around deck hardware is not only annoying, it is probably damaging your boat. With today's effective sealants, making deck hardware watertight is not all that difficult. Here's an excerpt from an article from Don Casey to help you avoid a costly repair.
Special Precautions for Cored Construction
Most decked boats are constructed with a wooden core in the deck to stiffen it. Smaller powerboats use a similar construction technique for the transom. If water is allowed to penetrate, rot in the core is the usual consequence. Repairing saturated or rotten core is a very big job, the cost often exceeding the value of the boat. More boats "die" from core problems than from any other single cause.
Water would never reach the wood core if we didn't drill holes in the deck (and transom) to mount hardware items. But we do, and as good as marine sealants are, it is a high-stakes gamble to depend on them to keep water out of the core. Anytime you bore or cut a hole in the deck, seal the exposed core with epoxy before mounting any hardware. If you are rebedding old hardware, be certain that the core has been properly sealed, or follow this procedure before reinstalling the fitting.
- Drill all fastener holes oversize. It isn't necessary to oversize cutouts.
- Remove all core within 1/4" of the hole or cutout. A bent nail chucked into a power drill is an efficient tool for chipping out the core. Vacuum the pulverized core material from the cavity. Whatever you can't remove will act as a filler.
- Saturate the exposed core with epoxy. On horizontal surfaces, seal the bottom of the hole with duct tape and pour catalyzed epoxy into the top. When the cavity is full, allow a minute or two for the unthickened epoxy to saturate the core, then puncture the tape and let the epoxy run out back into your glue container. For vertical surfaces you will have to inject the epoxy into fastener holes. Use a brush to "paint" the core around cutouts.
- Mix colloidal silica into the epoxy (the same epoxy you have already poured through the holes) to thicken it to a mayonnaise consistency--stiffer for holes in vertical surfaces. Retape the bottoms and fill each cavity level with the deck.
- Allow the filler to cure fully, then redrill the mounting holes through the cured epoxy. Sand and clean the area that will be under the fitting and you are ready to bed the hardware as detailed above.