Source? Me. I've personally assembled thousands of things using 5200 to seal, for a boat company, that Sea Fox bought out, and I have personally worked on a dozen different boats using it. And have deconstructed probably 1/2 dozen-same story.
Thru hull fittings are usually always sealed with 5200. Not used as an adhesive. The thru-hull's are secured with a nut. There is no use for an adhesive.
Some of the higher end sport/deck boat companies use it on ANY hole drilled thru fiberglass, even above the water line.
Anytime a thru-hull exhuast is installed, it is sealed with 5200. Same deal. Things are thru bolted thru the side, or transom, and SEALED with 5200.
Around bolts for swim platforms, drain plugs, livewell pumps, freshwater intakes for A/C, thru-hull transducers, the list goes on and on.