Well I got one of these new fangled Chevys(1988) that only needs sparkplugs after 100,000 miles. So after I replaced them in 2002, I figured I was good for another 14 years. Sure enougth, just seven years and 34,000 miles later, the truck starts running rough. I'm thinking it can't possibly be the spark plugs since I just replaced em a few years back, so I brought it by my more mechanicly experienced freinds house for a listen. He checked the exhaust temperature at the different cylinders with an infrared device, and noted that the exhaust leaked so bad it was hard to hear much of anything. After I explained that there wasn't much point is fixing the leaks since it passed the anual inspection in spite of them, he suggested that most engines run better with new plugs in spite of what Government Motors claimed. I decided to replace the plugs, and since I'd have one end of the wire off, I'd replace the wires for good measure, and since the wires were off the cap, it wouldn't be much more work to replace the cap and seeing that the cap was off anyhow, I figured on replaceing the rotor too. Well, all those parts were so cheap at the store, I picked up some new oil and a filter and since I was doing all this preventitive type maintainence, I aquired a new air cleaner and a p.c.v. valve to boot. I kind of hated to waste the good oil that was in the crankcase, but even if it wasn't worn out yet, it was still pretty dirty after 34,000 miles, and a new oil filter is always a good thing. While I was at the parts store, I noticed I still had some checks left in my checkbook, so I bought an upper radiator hose too. The left bank of plugs came out fine, and all but one were in good shape. Happily noting that I had most likely cured the problem, I proceeded to the right bank. The first plug broke right off, but I was able to remove the remainders with a large easy out. The second plug was more cooperative, but the third one snapped, and left a good portion of insulator in the head. Needle nose pliars and the easy out resolved that situation, bringing me to the moral of the story, if you live near saltwater, alternate the direction in which you park, elsewise your spark plugs will not rust evenly.
Your mention of the salt water reminded me of when I was in the Sea Scouts as a teenager (1960's). We had a nice 63' boat in Berkeley Harbor, and I spent a lot of my time there. Some folks lived on their boats in the harbor, and parked their cars along the breakwater that protected the harbor on the west from the San Fransico Bay. The west wind almost always was blowing in through the Golden Gate across the bay, and white caps and salt spray were the norm. The breakwater was narrow, so they could only park on one side, and they most always parked facing north. Some of those old Volvos and VW's and whatever they were hardly had anything remaining of their left sides after being parked for years directly exposed to the ever-present salt spray. Could't even tell what color some of them used to be. The right sides didn't look half bad on a few of them, though. Rick