Champion 821M has a copper core center electrode whereas the 821-1 has a nickel core center electrode. The copper core electrode has better resistance to the modern ignition system and better RF (radio frequency interference) resistance. The copper core plug will withstand the heat better than the nickel core plug and should last a bit longer, but in a boat motor, how long they last is meaningless unless you do not change them every year. The copper core plug is higher priced than the nickel core plug, but your motor will be able to run good on either one of those plugs.
