Go with what the manual calls for and make sure it is a marine grade plug.
It would be helpful to tell us what engine you are talking about. As a general rule it takes more voltage to fire a platinum or iridium spark plug versus a copper electrode spark plug. Platinum and iridium are used to reduce maintenance costs by having a spark plug that will last 100,000 miles. In fact, an old outboard engine that uses points will run better with an old-fashioned standard spark plug because it takes less voltage to fire the spark plug.