Sorry, I couldn't resist!
I am no expert, but I am sure learning a lot!
Here's my understanding... anyone: feel free to jump in with more / better info!
We are talking about the difference between cables designed to be placed in a dry environment, like inside the walls of your house, vs. marine cables designed to be installed in areas that have water, fuel, and oil present, all the time.
The stuff I used is marine cable, the copper wires inside all the insulation has been 'tinned', coated to help prevent moisture from getting into the wires themselves and corroding the wire, then the insulation that covers that attempts to remove all pockets of air, so the moisture has no place to pool, and start corroding the wire.
All that is covered by a coating (PVC in this case) that is more resistant to fuel & oil, etc.
In the description of the link you sent, they stated "Higher grade copper will offer less signal resistance and have less tendency to corrode or tarnish underneath the jacket."
Because the marine cable is subject to a harsher environment, the corrosion is exactly what we are trying to prevent... the marine cable description states:
"Resistant To: Acid, Alkalis, Abrasion, Flame, Gasoline, Oil, Ozone, Moisture, Fungus"
They both state they have a PVC jacket, so the difference may be the tinned wires, as well as the process for applying it to remove air pockets?
I decided to not to cut corners here, as I just wanted it to work, and not have to fiddle with it 5 years down the road, after it sat in the bilge, etc.
Went with 4 wire bundles for ease of installation, routed 2 cables instead of 8 wires.
I bought 50 feet, as I was not exactly sure how I wanted to route it, ended up running 2 strands from dash diagonally across the boat to the rear port side of boat.
I ran it under the floor, across & above the gas tank.
I ended up with about 14 feet left over, so figure about 13 feet per run in this case.
They also make a marine 12/4 cable that is quite a bit cheaper, and you can buy it buy the foot.
Free shipping, and they have a 12/8, too... run 1 cable...
I bought some 16/2, 14/2, and sealing shrink wrap from the above link too, very happy with all of it.
Yes, I could have saved some money, routed the wires differently, used speaker wire, etc., but I do plan on keeping this boat, and I wanted to do it right the first time, didn't want an unmanageable mess of wires all over the boat, and really wanted it to be reliable!
I would rather jump in the boat, turn the key, and go.... then have to mess with loose connections, etc. constantly...
I also thought about mounting a relay box near the pumps, then the heavy wire runs would be very short, but this would complicate my setup, and not work well with the switches I had already bought (came with the reverse-able ballast puppies)
Hope that helps.