Likely the slave solenoid on top of the engine as Dockside said (cross your fingers). Try jumping at the solenoid as suggested. If it still won't turn, have someone start it at the helm while you take a voltage reading at the battery. If there is minimal voltage drop, the starter isn't trying to engage. Check the starter solenoid (on starter) by jumping it. If there is a noticeable voltage drop, could be a bad connection somewhere or have a listen to hear if the starter is actually trying to turn. If you hear a hum, pull the plugs as suggested by Chiefalen. Have someone at the helm crank it over while you watch the sides of the engine. If it cranks and nothing comes out the spark plug holes, you have a starter, battery or wiring issue. If it cranks and water comes out of the spark plug holes, you've ingested water, and that's why the starter won't turn. Either an exhaust manifold leak or missing exhaust shutters.