did you check the connections at the main plug in power plug?? try giving it the old twist and jiggle trick and try the ignotion again..

Have you checked the neutral saftey switch?

Maybe the starter is bad or alternator is bad. Check both of those as well, and ensure all connections are tight. Anytime ive ever had a issue like this, its been the batteries, starter, alternator, and a loose connection. Im sure youve tried these but dont overlook them

Check the 20amp fuse before the ignition switch.

I am not aware of any fuse prior to the ignition switch. All of the other items were checked and OK. I have not been to the boat since Friday so I have not even attempted to jump the slave solenoid.
I do know via testing there is 12+ volts at both sides of the 50 amp, 12 Volts to at least one side of the solenoid [i do nto remember the result of the other side of the solenoid]. The other known is there is no power to the ignition switch and nothing occured to have anything 'break'. A fuse is likely but I cannot find one. I was working on the stereo and amp between the last time the engines ran and this problem.

I will be at the boat Thursday and will check out the solenoid, and try again to locate a fuse. The breakers under the helm are not tripped and do not have anything like ignition. They are all accessory type.

Thanks

Follow the large red wire from the ignition switch backwards. Should be a fuse somewhere in that line. Might be an accessory fuse panel under the dash that is powered by that hot wire from the Alt/starter/main breaker that holds the ignition switch fuse. Do all your other acc. work?

The items on the helm [bilge pumps, blower, lighting, etc.] function. the only NON working items that I have detected is the port engine. To me the odd thing is no power to the Ign. switch. I have used my multi meter on the helm area and the wire that carries power to the switch is dead. The same COLORED wire that carries power to the ST engine switch is 12.5V at the switch terminal.
The issue is from the 50 Amp breaker to the switch.
When I get to the boat this week my plans are to:

Look for an in-line fuse for the ignition switch

Try jumper wires on the slave solenoid

Even try a jumper wire from the 50 Amp to the ignition switch as a last option

The solonoids have nothing to do with power going to key switch. If the slave solonoid has power to one side, jump from that post to the small yellow one and engine should crank (You should only have power on one side unless it is cranking engine) . If it cranks then engine compartment power should be OK. Put a jumper wire from one key switch to the other (on red wires. Try to start it. If it starts just follow the red wire back to engine to find the break. Make sure to check your kill switch also(some boats are wired with pos. going to switch and then to ignition) If it is wired this way put a jumper wire across it and try to start. If its not wired this way disregard this step. Just follow the red from keyswitch back and you will find it.

I will be at the beast tomorrow and will perform the diagnostics mentioned. Makes perfect sense. The part that does not is why I am unable to locate a fuse and what caused this to happen.

Thanks again for all the replies.

i dont know if this helps but i thought i would add a suggestion, on the the 502/mag/mpi there is a fuse block on the starter???in my manual they just call it a 90 amp fuse system???i belive the wire that goes from the slave selenoid to the starter color might be red/yelow???