Did this a few years back. Drilled and tapped the hole..about and inch diameter (I'd have to double check). Bought a piece of threaded brass (threaded both inside and outside). Also bought a brass plug that would thread into the inner part of the piece.
I then put epoxy resin with some cilica beads mixed in both in the hole and on the threads of the tubular piece of pipe (this is the one threaded on the outside and inside. Screwed it into the hole in the boat and let it set up. Next day I screwed the brass plug into it to check...all good. Removed the plug and faired around the edge of the tubing with epoxy, sanded and was done.
When the boat comes out I screw out the plug...and back in before launch....I'll go take a couple of pics in an hour or so.
OK got the pics.
PIC 1. In the first one you will note that the tube/ pipe is threaded on the inside...the outer part of the tube is threaded as well and screwed into the boat. That is epoxied into the boat so that it can't come out...even if hit.
PIC 2. Direct shot looking into the boat. You can see the opposite side of the bilge. Note that the bottom of the hole is at the bottom and lowest part of the bilge so that it can drain completely. Can also see a ground wire going through there
PIC 3. Plug that screws into the pipe. It screws in all the way until the square portion is all that sticks out. "Pipe thread" so it doesn't require any sealant and it doesn't leak.
PIC 4. Where the hole goes on a C&C 30 .... that's the lowest part of the bilge as it's a little deeper under the mast step