A good marine sealant for bedding deck fittings must be waterproof, of course, but it must also be flexible, UV resistant, and, ideally, chemical resistant (fuel, bleach, and other solvents do find their way on deck occasionally). It should not be so strong that the deck hardware can't be removed if necessary, or so tenacious that it leaves a residue that prevents other sealants from adhering. From an aesthetic perspective, it should resist dirt and not age in an unsightly way.
The table below summarizes how the various adhesives line up against these criteria. Strength is measured in psi. Flexibility is measured by elongation, or the amount the material can be stretched, as a percentage of its original length, before it ruptures. When bonding two different materials together, and then subjecting the bond to movement, elongation allows the seal between the two materials to stretch to accommodate the differences in flexibility. A good sealant for bedding deck fittings has relatively low tensile strength and relatively high elongation.
The high tensile strength of the medium- and high-strength polyurethanes makes them excellent adhesive sealants, but less than ideal for anything that might ever need to be dismantled. In addition, the polyurethanes will soften when exposed to fuels, solvents, and particularly acids, so they're not the best choice for use on the deck. At first glance, silicone seems a better compromise, with lower tensile strength than the medium- and high-strength polyurethanes, but similar elongation and better chemical resistance. Unfortunately, the adhesion of general-purpose silicone sealants is often fleeting; you won't have to look far in any marina to see silicone "worms" dangling from port-light frames. Worse, silicone leaves behind an all but permanent coating to which no sealant, including fresh silicone, will adhere. Contamination from some earlier silicone sealant application is often the undiagnosed cause of premature bedding failures. But silicone cannot be avoided altogether as it is one of the few sealants compatible with all plastics.
The remaining sealants all have relatively low tensile strength and high elongation, and any of them can be used for bedding deck fittings. The low-strength polyurethanes have less holding power than their more muscular siblings, but share their susceptibility to chemical attack and incompatibility with plastic (except Sikaflex 295 when used with a primer).
Butyl tape is an old solution that's becoming popular once again, and it has a lot to recommend it It can be used to bed all types of plastics, unlike polysulfide, which attacks plastic, and polyurethane (with the exception of Sikaflex 295), which loses its grip due to plastic outgassing. More about Butyl tape on Thursday.