Put both switches on both and see what happens.

Wouldnt hurt to put a voltage meter on them either. The 496's require something like 12.4 volts for the ecu to be happy.

I did the both thing! didn't work! starboard starts fine. but port motor just makes the one click.

What is your voltage on each battery? You might want to throw a charger on them before you do anything.

If they test OK:
Switch the solenoids on the ECU bracket from one motor to the other.
(they are the 2x2 inch little black boxes)

sounds like the solenoid o the starter is bad to me

A single click each time the key is turned? Does sound like a relay or solenoid but have you checked the main ground on the back of the engine to make sure its tight?

Mercury uses two solenoids in the starter system. The click you are hearing is the secondary solinoid mounted up by the 50 amps re-set circuit breaker. The starter has a solinoid mounted on it that gets it's signal from the secondary solinoid. It is very common for starters to corrode up over winterization storage. Most marine starters when I take them apart are a rust ball of parts. You could try hiting the starter and it's solinoid with a hammer, LIGHTLY, then try it. If it starts you should consider it a warning sign the starter has corrosion issues.

The single click is coming from a relay mounted on the ecu bracket? not the solenoid on the starter? what does that mean?

IM gona try to put some love taps on the starter when one of my skinny employees gets here! lol

It means the secondary relay is working, that sends a 12V signal to the starter mounted solinoid. Also be sure the main battery wires are tight on the starter lug while your beating on it. Good luck.

Thank alot for your help once again guys!! It was a stuck solenoid on the starter!