Where are you putting the switch? If on the console inside the pilot house; no sealing required. Outside; most areas are not cored. If it is cored and a vertical surface, mostl likely you will be fine by just coating the edge of the core, and over drilling the holes for the screws and then filling with a thickened epoxy, then drilling a new pilot hole in the center or the plug.
Is the live well going under the "rear seat". If so, is it plumbed, and is there any new lining being put in place?
I have used two tools for drilling out for rectangular holes. One is a Dremel tool, or Rotozip, with carbide bits. You can also drill the 4 corners and use a saber saw if you have enough access. Two types of blades can be used: The carbide "glass cutting" or a bimetal moderately fine tooth blade. I have flexible shafts available for both the Rotozip and the Dremel tools. That allows getting into tight places. Often using a sanding drum to clean up the edges will make a cleaner cut.
If you have a large plate backing the rocker switch your might be able to use a hole saw. I have bi metal hole saws. I have almost every size from 3/4" up to 6" collected thru the years. Only a couple of sizes have worn out--and my collection dates form the mid 1970's including a period of building fiberglass boats...
Always run a drill bit backward for a few seconds before going forward. Tape the edges of the glass and cut thru the tape to avoid chipping.when using any saw.
You might try the Rotozip on some scrap material before you engage the full fiberglass cored laminate.
