One of the mounting points of the guard rail on the bow is leaking through the bow into the cuddy when it rains hard.

I can't tighten it down because the roof of the cuddy is covered in hard carpet - meaning i can't peel back the edge to access the nut to tighten.

Do i cut a hole in the carpet to access?
Suggestions welcome.

Boat is 1989 Baja 240

try to grab the nut through the carpet with a set of pliers or better yet vise grips....maybe wrap the tips with some electrical tape or masking tape not to stain or discolor the carpeting.....then either tighten it up or unscrew the bolt with someone below holding onto the nut and goop a bit of silicone in there and tighten it back up.......if that don't work or cant be done i would just cut one slit in the carpet big enough for you to peel it open to fit a socket in there...you should be able to glue it back together without much trouble....

what is 5200?
available at my local home depot?

No....its made by 3M i believe and avail at BoatUS or West marine....or local Marina should have some too....get the "fast cure" stuff....red lable on the tube....the regular blue lable can take up to 7days to dry...

Like Wally said. Cut a slit in the carpet and remove the bolt. Reinstall it with silicone. Dont turn the bolt from the top. Have someone hold it from outside while you tighten it from below. Turning the bolt will ruin the seal.

Rick