When Edwin and Garvey took Cole back to the island, what did Edwin take out of the boat's engine in case Cole decided to try to steal the boat and leave the island?

From the text:

Cole walked to an old plastic cooler and pulled out raw hot dogs.“It is…here.”When he saw the two men scowl, he added, “Just be glad I didn’t take the boat while you were gone.”
“That would have been a real trick,”Edwin said.
“What do you mean?”
Edwin reached in his jacket pocket and pulled out a spark plug.“Engines don’t run without spark plugs.”