Saltwater and electricity don't get along, or rather they get along too well, causing all manner of corrosion and non-conduction problems for the sailor. Keeping the electricity flowing between the lines is the problem.
One of my first published articles, in 2010, involved making up over 200 crimp connections and keeping them in a salt spray chamber for a year. I've also had boats in the water for 30 years and observed the result of factory, PO, and personal errors. What I have learned:
Flat trailer connectors (available in 2, 4, and 5 pin) are cheap, reliable if packed in grease, and a new one can be crimped on in minutes if you leave a loop of wire. They have been the standard for 75 years, so I'm not expecting change. It also proves they work. My choice for rotating masts, tiller pilots, and solar panels.
67. THHN Machine Wire Copper Wire vs. Tinned Wire. Heresy you say. In fact, both are approved by the US Coast Guard, and both survived my 1-year spray test for Practical Sailor Magazine with zero failures. There are differences in corrosion. Do no use finely stranded (like lamp cord) copper wire without tin plating; i will not hold up. Additionally, when it comes time to repair copper wire, the corrosion may well have traveled far up the wire. However, in dry areas of the boat, does the extra protection mean anything? If the breaker panel, for example, goes underwater your going to rip it all out anyway. Many quality boats use lots of non-tinned wire without complaint.
Do use tinned wire to all exterior lighting, around the engine, and in the bilge. You won't regret it.
Pitch the cheap crimpers in the trash. And I'm cheap!
An inexpensive pair like this works for me. Just make sure they are adjustable (star wheel near "Titan" label). I have a fancier one at home, but I'm not convinced it is better for occasional use.
68. Ratchet Crimpers. Remember how I said I did over 200 crimps for a PS test program with no failures? The key is to throw away any non-ratchet crimper. That's right, pick it up and pitch it before you do any damage with it. They just are not repeatable enough. Instead, get a ratchet crimper, adjust it to the brand of fitting you will be using (no Harbor Freigth stuff, but the Home Depot stuff is fine), and make a couple test fittings. Clamp the eye in a vice and try to pull it off the wire. The wire should not pull out. In fact, either the wire should break or the fitting should tear.
69. Grease. Forget the sprays. They don't hold up in marine conditions. Forget conductive grease. Many can cause dissimilar metal corrosion and "conductive" in the sense you understand is pure myth. They are insulators except at extreme voltages, where they can bleed off minor charges (never use conductive grease on antenna connections or spark plug wires). Vaseline can work, but it melts easily and has inferior anti-corrosion properties. Dielectric grease is good for sensative electronics and antenna connectors, but in general, good old waterproof grease or Lanicote is the correct choice. Use this on all studs, terminal strips, batteries, and all mechanical contentions (not required inside the crimp).
My favorite is No-Ox-Id Special A. It has been top performer in all of the extreme exposure testing.
70. Separate Heat Shrink. Not technically about equipment, but sort of. Instead of buying pre-insulated crimps with adhesive lined sleeves, buy the heat shrink separately. Not only is it much cheaper, the odds of the insulation sleeve being damaged by the extreme pressure of crimping are very high, perhaps over 50%. So instead of buying the over prices all-in-one crimps, buy economical insulated crimps, use them as-is for most cabin applications, and then cover them with separate lined tubing only when used on deck or in the bilge. This also reduces the required inventory of crimps.