We could do an endless thread just on 3M adhesives and sealants.
3mtm-adhesive-sealant-brochure.pdf
They put the word "Marine" on a line of products, probably as marketing tool but hey, it draws our attention.
We get caught up in the 5200- 4200 debate when they have a lot more products.
Look at the tensile strength of 5200. It is by far the strongest stuff they have. It is meant to hold things in place.
You can glue the deck to the stringer with 5200 and no fasteners and it would be better than the polyester resin adhesive that the used when they built the boat.
I would use 5200 on thruhulls below the waterline because the grip you get from a big threaded nut is crap. Will you destroy the part trying to remove it..yes... how often do you remove thru hulls expecting to put them back?
It is slow cure...even the Fast Cure is slower than others.
I would use 4200 or better 4000uv for bonding/sealing deck hardware..drains, rodholders, anything you screw into fiberglass that won't need to be removed in years.
I have a seam of 4200 on my gunnel that looks like crap...yellow with black spots. I know now I should have used 4000uv.
For things that get removed often (deck hatches over fuel tanks should be removed OFTEN) I would use quality exterior silicone