I have an 1990 454 Magnum.

When starting I get the classic "click click"

New battery, no luck.

New cables, no luck.

Rebuilt the starter with new solenoid, no luck.

What's left??

Any suggestions?

The only thing I can think of is the ignition switch.

P.S. Also, once I get it started it "usually" starts instantly

Short in the wire going from the ignition switch to the solenoid or its the ignition switch. Heck, you've replaced everything else.

starter relay?

It most likley the Slave Solenoid for the starter. Not the one on the starter. If that is not it, check your ground connections.

What he said.
The slave solenoid is a common problem on Mercs.
The longer the boat sits without use the worse it can get, especially in humid conditions.

The solenoid is a "self cleaning" design so the more you use it the better it gets, usually clicking it a few times will clean it off so it will work fine, until you park the boat again for a week or 2...

They are inexpensive so just replace it...

Could still be ignition switch or kill switch or like said before solenoid.
Easy way is to get a check light and have somebody crank it and check at the different locations for + power. Start at the starter. This way you can see if your getting power to the back of the boat.

You are getting power to it, just not enough. Check all connections real good esp. the GROUNDS....

I missed the part its clicking. There is power to the starter. Like said before check Connections.

The slave solenoid makes a clicking sound too, even if it isn't working. Most likely the slave solenoid and is certainly the first spot to look based on what you have done already. Short across the big poles with a screwdriver. That should wake er up.

Also, since you said that "Also, once I get it started it "usually" starts instantly" is another indication of a bad slave solenoid.

Really easy to test it and then replace it.

Don't go to the dash until you have ruled out the slave solenoid.

Power to the starter motor is only one piece of the puzzle. Power to the starter solenoid comes through the slave solenoid from the + terminal on the starter motor, the same terminal the battery is hooked to.