just out of curiosity,
you do have to turn the keyed switch all the way to the start position to start the motor and the spring in the switch turns it back to the on position when you let go of the key?
Generally when a keyed switch fails it just doesn't actuate the starter solenoid.
My company was a service/warranty center for several equipment manufacturers who's equipment used ignition or keyed switches. I do remember one case where the switch spring quit working and you had to turn the key back one step to the on position from start.
DC electric can be tricky. For instance you could have a "short" in the wiring somewhere which could make it seem the keyed switch is stuck in the start position and or could even make the problem come and go away randomly.
You can disconnect the wire from the solenoid to the starter motor and then turn the key to start and you may be able to hear the solenoid click. Don't hold the key in the start position more than a few seconds though the click should happen immediately when the key reaches the start position. Do that a couple times to see if it keeps clicking. If it does then it's not the key switch because it is energizing the solenoid when turned to start and stopping when you let go. If it does not keep clicking when turned to start but does at least once then the switch could be sticking in the start position and should be tested with a meter.
As I mentioned DC can be tricky, if checked with a multi meter and it shows continuity in the starter circuit even with the key
not in the start position then remove that wire and test the switch again. If it still reads the same then the switch is bad. If it does not then you have a short somewhere.
It takes longer for me to type this than to trouble shoot your problem if I had your boat in front of me
