Spark Plugs: automotive plugs OK, as long as you boat in a freshwater environment and drydocked. More questionable for freshwater boating + wet slips; recommend going with marine plugs for the extra corrosion protection (which is the only difference between marine plugs and automotive plugs). If in salt water, marine plugs.
Starter. I replaced the starter in my Mercrusier 3-liter, 4 cylinder, inboard engine with an automative starter. Price for the marine grade starter at the local marina, $295. Price for the marine grade starter on-line, about $175 plus shipping. Price for the automotive starter at local auto parts store, $49.95. Information that I was given was that the two units were the same, but the marine unit is sealed better. By my estimation the engine comp in the boat is as dry as a car engine. Was told that i could seal any openings on the starter with gasket material, but I didn't. Because of lack of sealing, and the gassy env in the boat eng comp, I was told to make sure that I run the blower before turning the engine over. I did need to remove the old starter and bring it in to the auto parts store in order to get a suitable replacement. New auto starter is working great so far.