Whether you have to prime first and which primer to use can not only make a big difference in your paint job, but also in how long it lasts. Primers are designed primarily to promote adhesion for the topcoat to the surface, not just for hiding the surface with a consistent color.
Priming for 1-part paints: Most paint manufacturers recommend using their proprietary primer before applying 1-part paints because the adhesion for 1-part paints is good, but more subject to lifting, cracking, or peeling from use, exposure, and continuous water submersion than 2-part paints. 1-part paints need a slightly rougher surface to bond to because they do not have quite the mechanical bond of 2-part paints. Epoxy primers can generally be used under most 1-part topside paints, but it may be an excessive cost when there is no benefit for the paint. Some 1-part paints will stick well to sanded, clean fiberglass or wood, and will state that priming is recommended, but not mandatory. History commonly shows that a properly primed boat will have better paint adhesion than one that was not primed.
Priming for 2-part paints: 2-part topside paints are seen as a different animal when it comes to priming because they show to best advantage and adhere best when the surface is sanded very smooth. This is because 2-part paints have much better mechanical adhesion properties than 1-part paints, which require a rougher surface for adhesion. Most 2-part paints are more sensitive to contaminants and substrates, so, for best results, they require a 2-part epoxy primer that has fully cured to an inert state, and that's been sanded smooth. Epoxy primers are ideal under 2-part paints because they're usually designed to be sanded down to very fine sandpaper.
Priming metals: In almost all cases, metals need to be primed before painting. Depending on the metal, primer, and desired topcoat, the surface may need to be etched, or a special metal primer, such as a chromated primer, may need to be applied before applying the topcoat's recommended primer.
In any case, it's always prudent to follow the specific paint manufacturer's recommendations for priming because manufacturer's want customers to have the best end results and satisfaction.