Both a dremel or a router would work depending on how much room you have to work with. The problem is going to be proper location of the hole. I would mark the location inside the boat using the chainplate as a guide. Then use an extra long 1/8 inch drill bit to drill holes at each end of the mark. Put a little grease at the midpoint of your index finger and thumb, and bend the bit to guide it alongside the bulkhead. This way you can ensure that the hole you cut is directly above the bulkhead and the correct angle through the deck.
Edit: Make sure you bed the chainplates properly with butyl tape.