Because boats are operated on the water and often have wet or very humid places in which electrical wiring and electrical connections are located, those electrical connections are prone to forming oxidation between the two metals of the electrical contacts. Because the voltage in the circuit is relatively low, typically not much more than 12 to 14-Volts, when even an extremely thin layer of insulation (the oxidation) forms, the current flow will be completely stopped or a significant resistance will be formed, mostly due to loss of contact area between the two conducting metals.
A good method to restore an electrical connection in which oxidation has affected the flow of current is to disassemble the connection, then remove oxidation from all the connections being made at that circuit node. Typically in electrical connections the wire is terminated in a ring terminal connector which is made of copper and plated with a layer if tin. The purpose of the tin is to cover the copper completely, as otherwise bare copper is extremely prone to turning into an insulator instead of a conductor if water reaches the copper and turns it into an oxide of copper, which is a poor conductor.
The connection surface of a ring terminal connector should be cleaned using WD-40, and if necessary, lightly wet-sanded with 400-grit emery cloth and WD-40. After cleaning, wipe away any excess WD-40 using a clean cloth.
Do not use a steel "wire brush" on electrical contact surfaces, as the steel will tend to scratch into the surface of the tin coating and reveal bare copper. The scratches also create small depression which are perfect for oxidation to begin to form. If you need to use a brush, as might occur with a large terminal post, use an old (but clean) tooth brush and use WD-40 as the cleaning agent. After cleaning wipe away any remaining WD-40 using a clean cloth.
After the connectors, including the connection surfaces of a terminal post or binding post, have been cleaned, the connection should be assembly dry and properly tightened.
Do not smear the contact surfaces with any sort of grease, as grease is generally an excellent non-conductor. The often cited "dielectric grease" is actual an extraordinarily great non-conductor, and the last place you want to introduce a non-conducting intervening layer is between electrical connector surfaces. Other problems with using grease are it tends to attract and accumulate dirt, and if the temperature at a connection rises--which is likely to occur when the resistance of the connection increases--the grease will melt away, often making a further mess to be cleaned.
To protect the re-assembled electrical connection from ingress of water, use an aerosol spray product that is specifically designed to be used as a protectant for electrical connectors, such as Boeshield T-9, CRC Battery Protector, Evinrude DPL Anti-Corrosion Lubricant, or similar products.
Regarding using grease, there are certain situations in which using an electrically non-conducting grease can be helpful. For the rubber boot on spark plug connectors, use of a dielectric grease to create a seal between the ceramic body of the spark plug and the rubber boot is often done in order to prevent water from entering the boot. A very light layer of thin dielectric grease can also be helpful in preventing the rubber and ceramic from forming a very strong adhesion which could make removal of the rubber boot from the ceramic plug surface very difficult. Only use dielectric grease, which is formulated to have extremely good properties as an electrical non-conductor and will inhibit any tendency for the spark voltage to leak down the side of the ceramic insulator.
Another possible application of grease can be to provide a lubricant to small seals designed in certain mating connectors. Before apply any grease to a sealing surface in an electrical connector, consult the manufacturer's literature to see if use of a dielectric grease on the seal surface is recommended. Some seals may not benefit from grease or could be harmed by grease. But, again, the grease is never applied to the actual metal electrical contact surfaces before mating or assembly of the connection. If grease is used, only a very small amount is needed.