Use dielectric or Vaseline....metal on metal makes the connection - the non conductive gel just stops the corrosion...been doing it for years and it works.
100%, slather grease on any exposed metal, especially anything copper. The only way to prevent corrosion in this application to to keep 100% of the salt water out.
There's always a ton of debate around this topic. It's true adding grease/vaseline/fluidfilm to an electrical connection will result in slightly worse contact, higher resistance. However, although a dry connection is the best electrical connection, a dry connection only stays good until corrosion starts. After that the connection gets worse, higher resistance, maybe gets hot, stops working, if you're lucky one day even starts a fire. Connections with exposed copper, ends of wires into connectors etc are the worst. Tin plated parts do seem to hold up well.
A connection that's fully coated in grease or whatever waterproof goop you want will be slightly less good from day one than a dry made connection, but it will stay slightly less good for a very very long time, where the dry made one will turn to absolute crap from corrosion pretty quickly if there's any salt water exposure. Unless you get very high quality plugs like the Belkins mentioned already here, dry connections with any salt water exposure are going to turn to crap.
My philosophy is grease the heck out of every connection and be confident they will good enough and remain reliable for a long time. I spray everything with CRC corrosion inhibitor. It seeps into all the crevices and sets up into a thick waxy coating. I find it sticks around longer than fluid film and gets better coverage than grease. For the scotty plugs I stick the straw from the spray can inside the plug boots and spray liberally.