There are many choices when it comes to choosing a caulking or sealant for a sailboat. First you should recognize that there is a big difference between sealants such as caulk and adhesives. Adhesives must work harder to hold things together and therefore are tougher. Caulks, like silicone, latex, etc, are used to keep water out of a place. You can use some sealants for adhesive purposes. For example, silicone may be used to adhere things you may want to take off later.

Latex or acrylic latex and siliconized latex caulks are inexpensive and dry quickly. The disadvantage is that over time they may grow mold and shrink after use. Latex caulks are not well suited for use on a sailboat

Rubber or Elastomers for use on a boat may be be Isoprene, Butadiene, Nitrile or Polychloroprene.  Rubber based sealants will adhere to almost any material and are resistant to oil and gas. The main use of these type of sealants is in fuel systems and tanks.

A good sealant for use on a sailboat is silicone. They cure fast, adhere to plastic and metal and may have a life of twenty or more years. Silicone that uses ammonia to cure instead of acetic acid causes less corrosion. Silicone is horrible to have around wood that may need to be varnished or painted. The slightest bit of silicone may prevent varnishign of an area no matter how deep you sand. Keep silicone sealant away from areas to be painted. Silicone is well suited to through hull fittings made of plastic.  You should not rely on silicone as an adhesive for critical parts.

One of the best sealants to use on a sailboat are polysulfide adhesive sealants. Polysulfide adhesive is a good choice for chainplates, teak deck seams, etc. Polysulfide sealants cannot be used with most plastics due to the fact that they contain solvents like Xylene that degrade plastic on sailboat parts. An advantage of polysulfide sealants or adhesives is that they can be easily painted.

Polyurethane adhesive sealants bond to many kinds of substances like fiberglass and metal but are more subject to being degraded by oils and gasoline.  You can use these kind of adhesives on hull to hull and hull to deck contacts.

Summary:

For bedding materials together consider using silicone or polysulfide. Avoid polysulfide with plastic. Keep silicone away from wood to be painted. For seams use polysulfide or polyurethane adhesive sealants and remeber that polyurethane can be very hard to remove.  For more info on what sealants are best for sailboats ask you local West Marine sales associate. They carry a wide variety of of sealants and adhesives and have a chart in the store that shows what kind of adhesives should be used where on a a  boat. Another great resource is This Old Boat which is a great reference for owners of both new and older boats.