When I try and start my boat (w/ 496 mag), I hear the warning beep, and when I turn it over, I can hear 1 relay click on the top of the engine. After that, nothing.

I had this problem 2 weeks ago out on the water, I came back and cleaned the battery terminals, and the boat started right up. I added a 3rd battery to the boat just for added assurance wired in parrallel. All stereo and accessories work properly. Now it is doing the same thing. What else could it be besides corroded terminals?

Is there a relay that goes bad?

Electrical start issues can be a few different items, but yes the 496 has a start relay as one of the four relays on the relay bracket. The cube relay that has a yellow wire with a red or back stripe coming out the bottom is the start relay and if its bad it will not allow the starter to crank. You could also have a problem with the connection at the neutral shift switch on the shift cable bracket at the right rear top of the engine. If the switch there thinks the boat is in gear it will not allow you to start. Try starting while pushing the tab on this switch in and out. Also check your connections down at the starter solenoid on top of the starter, both the red battery cable connection there and the yellow wire connection there also.
You can also clean dn check your various ground cable and wire connections on the engine and bellhousing as a bad ground acts just like an open positive connection-NO Power.
Hope this helps.

Best Regards,
Ray @ Raylar

Thanks so much for the suggestions.

I tried replacing that exact relay with another relay, and nothing changed. I could still hear the other relay clicking.

I took the tab on the neutral shift swift off (2 little screws) and tried purching this tab in and out as you suggested. From looking at the setup, it appeared the tab needs to be extended all the way to tell the motor that it is in neutral and allow the boat to start. Pressing it many times and trying it all the way out didn't work.

Where is the starter solenoid located? Is it on the back of the engine? What is the easiest way to access it? I have looked all over and from what I can tell, it is tough to get to in the back of the engine compartment.

Thanks sooo much for your help...I am desperate, I have a huge crowd of people who came into town who are all disappointed

The starter solenoid is on the top of the starter itself.
You should listen for the clicking of that starter relay mentioned above, if it clicks when you trun the helm key to the start position then obviously the start circut from the helm is working and it could be the solenoid or solenoid connection down on the starter. If you get NO clicking at that relay when the key is turned to start then it probably means the the starter circut is not working at the helm or from the helm. Check your safety Lanyard switch or the key switch itself. also check the boats helm cable where it plugs onto the engine in either a 10 pin push in connector or a 14 pin twist and trun connector.

Best Regards,
Ray @ Raylar

Thanks again!

I can hear the starter relay clicking so the start circuit from the helm is working.

Where is the starter physically located? I can't seem to find or see it. I guess my next step will be to check the connections there if I can access it...

starter is on starboard side,back of engine,way down next to oil pan,,,try tapping starter while trying to start,,,also make sure your starting battries are fully charged

Has the boat by chance ever had enough water in the bilge to get into the starter? That's a very common cause for starter failure down the road a bit.

Spent days tracing an issue sort of like yours but my starter would crank but no fire.. Loss of power somehere between motor and dash..Just started pulling apart every connector and found one in the dash in the far corner that when I cleaned the connectors it fired right up. IT looked ok but I guess was not making good contact.. Learned a valuable lesson.. Check all connections first..

2 weeks ago when I had it out, the first time I tried to plane out the boat this year(FYI, I am new to powerboating, long time sailboat owner)....the boat wasn't planing out properly so I backed off the throttle to re-evaluate my method (way to quick) and salt water came up the back and possibly got into the 4 rear engine bay vents. This is the only time I have any knowledge of any potential water intrusion into the engine bay. The same day, I was out starting and stopping the boat ~10-15 times before the LAST time I tried to start the boat and come in and that is when this issue began.

I wonder if because of my dumb error that caused water to enter those vents might have caused just enough corrosion at the starter/solenoid that by the end of the day, I would have this issue, and now a couple weeks later, more corrosion has occured. I know salt water is very troublesome.

So. It looks like the starter is the next place to check. It looks very difficult to gain access too. I was able to follow the + wire from the batteries up under the starboard rear side of the motor and feel what I thought was the starter. It did feel like some corrosion may exist under there even though the rest of the engine bay is spotless and looks brand new. It looks like I might have to remove both of the batteries on the starboard side to gain better access to see what I am dealing with... Actual starter replacement looks like it could be rough.

It was a major let down this weekend...but hopefully I can get this issue taken care of ASAP. You guys have been extremely helpful!