"""""This sounds like a PITA! I'd have to use an ohm-meter with a really long cable attached to the ignition accessory wire and then use the other test lead to probe all the engine wires until I find a match... Is there an easier method here that I need to know?""""
Yes troubleshooting is a pain in the *****, Why do you think it costs so much to pay someone else to do it?
The oil pressure switch is a on/off switch. When key is in run position 12 volts is present at pressure switch. When oil pressure is reached, switch closes and passes that 12 volts to fuel pump. One side of pressure switch should have TWO wires. One of those wires goes to pump, other should go to starter solenoid to supply pump with 12 volts when cranking. With out this the pump would not run when cranking/starting engine. Once pressure is present and pressure switch closed and engine starts and key is returned to the RUN position then 12 volts to pump goes thru switch ONLY.....
There cannot be any other source for this 12 volts to pump regardless of key position.
12 Volt supply to fuel pump sources-
1. When cranking, 12 volts comes from starter solenoid ONLY which bypasses oil pressure switch to start engine ONLY.
2. Once ~5 psi of oil pressure is generated oil pressure switch closes, 12 volts now comes from Ignition on/run ONLY (Purple wire)
If your boat ran good last season before you put it away for winter, what was done to it before you attempted to restart it for the spring?
Something is fishy here...............Did you replace anything? Did you mess with any wiring? Did you install something and wire it to the wrong place/power source?
Here is a standard wire schematic for dash board wiring. This is very typical for all boats.