Yamaha F225 - No Crank - Intermittent

Greetings.

New to me boat with a pair of F225 Yam 4 Strokes. One of them is sublime. The other, not so much.

It has an intermittent "no crank" condition when the key is turned to start the engine. If you release and try it again, it may or may not work. The situation is getting worse.

I am trying to determine if I have a switch problem at the helm, or a starter problem on the engine.

The battery terminals are clean as a whistle.
The batteries are fully charged.
The connectors on the engine are spotless.
The other engine starts perfectly, so that ought to rule out the batts, however, I have two rotary battery selectors and I have NOT tried using them in any configuration other than BOTH and I have not pulled them to check for corrosion.

I am considering swapping out the ignition switches at the helm, left to right and right to left to see if the problem follows. As of today, I have not tried this, and have no idea how difficult a process it is given all the circuitry involved at the gauges.

I do have the factory manual, but I am on the wrong side of the country to use it.

Any and all suggestions welcome and many thanks in advance.

Try jumper cables directly from battery to starter

Is there a click at the engine when you turn the key but it doesn't try to start? If no click, I would try swapping the key switches as you have already thought of. If you are getting a click, my first check would be all the battery cable terminals for that engine including those at the switches. They can look perfect under the heatshrink but be horrible once the heat shrink is peeled back. You can follow a procedure to test for voltages that will lead you to the bad joint if there is one, but comment on the clicking first.