I just bought a 2000 Baja 342. It’s has two 502 magnums MPI. The starboard engine was rebuilt 19 hours ago. The port engine has 760 hours and it’s original. I backed the boat in the water last week and the starboard started while the port did not. I had to replace the starter/fuel pump relay switch (very easy), changed the oil in the port engine and changed the lower units gear oil. Took it back to the launch and the port engine would start but now the starboard would not start. It will make a half turn and then seems to lock up. This happens each time you turn the key. I thought maybe it was a dead battery, so I replaced both batteries on that side and it still does the same thing. When I pulled the boat out of the water and pulled the plug it seemed to have gotten a lot of water in it. I assume it coming from the port engine some how while it was running. Any thoughts???

Maybe when U backed the boat in.. water came in through exhaust and locked engine? Are the flappers in exhaust good?

I don’t see any flappers. Does that mean the engine is ruined?? I’m kinda mechanically inclined. If told to do something I can do it and once I understand how something works I can usually figure things out, but I have called a mobile boat mechanic but he can’t come out for 2 weeks. Anything I can do for the locked motor or is it too difficult or shot now?

Did you hold the key on trying to force the starter to roll the engine or just not press the issue? If you held the key on hoping for a roll over you could have bent internals....

I would pull all the plugs, ground the ign. and then roll over the engine.... then put spray some oil into the cylinders if it spins....

no I tried to start it about 8 times. I wouldn’t say I laid on the key, but I tried to start it normally

Are you saying you removed the plug and there was a lot of water in the bilge, or pulled spark plugs and had water in engine?
sounds like you need a compression test to see if you have a short connecting rod now, and then find the root cause.

I pulled the plug in the boat. Not the spark plug.

How steep was the ramp? Did you submerge the transom before the hull floated off the trailer? I have seen water push up the exhaust once on that scenario.

Pull the spark plugs and see if the engine was hydro locked...very evident if it was.

So I pulled the spark plugs. Put oil in the cylinder turned motor over and had a lot water in one of the cylinders. Got water out, changed spark plugs fires up motor and it runs. A lot of “steam” coming from exhaust. While there is none coming from the motor that had no issue. I turned motor off and then restarted and it starts fine. Checked the oil and it seems to now have a bit of water in it. Will an oil change solve the problem? I had a machanic tell me to just run it and let the water evaporate out because it does not seem to be much at all due to the dipstick showing full but not over full. Any thoughts?

Change the oil for piece of mind. Get a set of "Salsberry Flappers" if you have straight thru-transom exhaust or internal flappers if you have angle cut thru hull exhaust.
Where in TN do you hail from?