Anyone who has ever turned a boat's ignition key knows the frustration of hearing nothing. Again, this is most likely an electrical issue - a low or dead battery, or a break somewhere in the ignition circuit.
Solution: Check the kill switch. Make sure the shifter is in neutral. Then pay special attention to the starter switch itself. Sometimes, a dash-mounted ignition switch will simply become loose in its fitting, allowing the entire switch mechanism to turn with the key. Fixing this can be as simple as getting behind the dash and tightening up a retaining nut or mounting screws. If the starter groans but won't engage, it could be a low battery, but it also might be a loose or poor connection.
Prevention: We'll say it again - inspect, clean and, if necessary, replace your wiring periodically. If you habitually drain the battery by cranking the tunes while at anchor, consider installing a secondary battery bank or one of those metering devices that monitors supply and saves enough reserve to ensure a restart.
And remember to always carry onboard: Screwdrivers with insulated handles; wrench set or crescent wrench. Happy and Safe Boating on what's going to be a sunny and warm weather Sunday!