A visual inspection of your boat engine's sparkplugs can help you diagnose problems. In this instance, a case of hard starting was the result of carbon buildup on the plugs.
Carbon is conductive and the electricity follows this less resistant path rather than creating a spark at the electrode. Carbon fouling is not the plug's fault, however, the outboard's owner had installed a plug that was too cool for the engine. ( In fairness, the plug was recommended by a store clerk as a replacement for the original plug the owner had asked for>) At any rate, changing to the correct plug had the engine starting and running beautifully.
For top performance, always make sure to replace any parts with those meeting the specific recommended by the engine maker.