Couple options, depending on the type of fitting. For anything that will not have an exposed sealant surface, e.g. mooring cleats, tracks, U-bolts, hatches, chainplates, etc., butyl (window glazing) tape is best. This is almost the same stuff as blu-tack and comes in black or white. You stick a sausage of it on, tighten it up, then wait a day (that's the important bit), and then tighten up again. Then cut off the stuff that has squeezed out with a putty knife. It never sets, so remains flexible forever. Completely waterproof and relatively easy to remove when needed. Downside is that any exposed butyl surface will remain sticky forever and thus attract lots of dirt, so you can't use it in situations where you need a sealant bevel or some such.
Medium strength PU based adhesive sealant (Sikaflex 291i) is commonly used for situations where you have exposed sealant, e.g. when filling gaps. You could use a pure sealant instead of an adhesive sealant, but the pure sealant often does not adhere to surfaces as well, which is why this stuff is so popular. It's also sandable, useful if you're using it anywhere near timber or want to varnish over it. There is a high-strength version (292), which you should not use unless you really really need the extra strength and don't want to take it apart again ever.
Then there is Arbomast BR, a butyl sealant in a cartridge that stays very flexible like butyl tape, but will (slowly) achieve a surface cure, thus not attracting dirt. Only place I've found this useful was on the stanchion feet on my boat, and for sealing leaking deck cable glands on another one.
For teak decks there is finally deck caulking, which is more rubbery, cures harder, has high UV and chemical resistance and is well suited for sanding, but even messier to apply than 291 and about twice as expensive. Common ones include TDC, Sabacaulk and Sikaflex 290dc.
Factors that influence sealant choices include UV stability (any exterior stuff that isn't completely enclosed), acid resistance (several common teak deck mistreatment products destroy sealants) and fuel resistance when used anywhere near fuel fittings.
Happy gooping ;-)
