While it would likely work fine, they use it on underwater telecommunications cables, I am not personally recommending it or using it for that. So, USE AT YOUR OWN RISK. It works great though for deck hardware. Even the hull/deck joint on my our boat was done with butyl..
Other than conventional wisdom, why?
My first expereince with butyl was sealing concrete pipe and sealing the upper and lower halves of precast tanks together. All of this has been in the ground for 25 years by now, and none of it leaks (leak-tested this fall). Of course, the amount employed (2 x 1-inch square cords) is much greater, though most of this squeezes out over time.
I know butyl has a weakness with petroleum and diesel in the bilge wouldn't be good--never is--but the installations I'm an discussing are in the oil/water separation facilities of a refinery. They are not immersed in oil, but they have been exposed to some emulsified oil the whole period.
It also would not provide back-up adhesive properties, should the mechanical fasteners fail.
I gather it is a combination of concerns. When I rebed, I may go that way, since I have no fuel in the bilge (tanks and engines in bridge deck) and the through hulls are in a bulkheaded compartment.
I will share this expereince with butyl:
This summer I had to re-glaze a hatch (happens, when you drop an anchor on it). The factory had bedded the hatch 14 years earlier with butyl; it had not leaked a drop and the remove/replace process was an easy 15 minutes.
It is not the only sealant I use, but it is high on the list.
