I have a much better idea! If any wood in that transom, drill the hole at least an 1/8" OVERsize for the fastener and drill thru. Then:
-Cover the lowest end w/ duct tape.
-Coat out and seal the wood w/ std epoxy
-Once 'kicked' (tacky) fill w/ thickened epoxy
-Drill thru for fastener body when fully cured
-Always save your mixing cups/application tools, to verify your epoxy had a full and complete cure/bond
FWIW the BEST BUY in epoxy is that from Raka epoxy, https://www.raka.com . Buy their $18 epoxy sample kit and get some epoxy w/ various hardeners (fast/slow) and fillers. Just a few bucks for postage and you'll get it in 2-3 days! This kit has ALL the DIY'r needs to use to effect small repairs.
In regards to sealants, I only use 3M 5200 below the waterline. If using my epoxy trick above, and above the waterline, I prefer LifeCaulk by BoatLife. It is cheaper than 5200, almost as tenacious, but the tube last YEARS - where an improperly closed tube of 5200 last months.
TIP - when using 5200, squeeze what you will use out onto a piece of plastic scrap and close/cap the tube WITHOUT letting any goop get exposed to air and get sucked back into the tube. This stuff starts to cure when exposed to air and is why the top of the tubes get all hard.
I will still use it only below the waterline, like for a seacock or water inlet scoop installation. Otherwise I like BoatLife's products much better!