High quality connectors like weatherpacks, metra-packs, or Deutsch connectors. Also, use di-electric grease on everything. Avoid placing harnesses in places that water collects and stays (channels and such). Don't mount relays upside down and water will collect in them (you have no idea how many times I have seen this).
All good suggestions by Philofab. Adhesive lined shrink tube over non-insulated crimp terminals has proven to work very well also. Liquid electrical tape can be used to seal switches, breakers, relays, lights, etc. Use rubber boots on switches and breakers.
Coil packs don't really care unless perhaps totally submerged, IME. Alternators same, just blow the mud out once in a while. Distributor... Hmmm. you'll have to refresh my memory. What is that again? oh ya, that problematic POS the OEM's did away with 20 years ago Vaseline or dielectric at the base, a remote vent, and frequent WD-40 or the like to displace any lingering condensation is about all you can do. Get rid of it, and you don't have to waterproof it!
The only thing about water (and dust!) you can garantee is that given sufficient exposure, it WILL get EVERYWHERE. I spend 6 months of the year with water on the ground, water in the air, and condensation everywhere else. Just slow it down where you can, and make sure you have good drainage! For computers and fuse blocks, silicon seal them into tupperware containers with air/watertight lids, and have some silica gel desicant inside to take care of ambient humidity changes with temp.