3M makes a variety of adhesives to be used on a boat. The three main ones are Marine Adhesive/Sealant 4000, 4200, and 5200. The 5200 comes in fast and slow cure. Essentially as the number goes up, the more permanent the adhesive. More permanent is not necessarily better, as you may need to remove that thing that you glued for maintenance some time in the future without stripping off the gelcoat.
Which one you should use is actually more complicated than just the adhesive strength, as it depends on what you are trying to adhere (wood to metal, metal to fiberglass, etc.), whether you need it in a color other than white, whether you need to use it below the waterline, etc.
There is a very nice chart of the 3M adhesive/sealants you should use for particular applications. We have a printed copy that we keep with the sealants. The 3M web site also has a similar chart. This article is also a good reference, and lists additional adhesives that may be useful, too, such as SikaFlex 291, 292, 295, and 296.
There is a stuff that will debond 5200. This describes how to use it.
Another terribly useful site when trying to figure out how to connect two items is This-to-That. You enter that you want to glue together glass to Styrofoam or ceramic to rubber, as examples, and it recommends the right glue for the job.
