I have a 2001 Bass Cat pantera 3 with a 2001 200hp Merc optimax. The problem I'm having is that the motor will start fine out of the water with muffs on, but when i try to start it at the lake with the lower unit submerged, nothing. After doing some research I think I've narrowed it down to one of 3 things; 1) the main leads from the battery to the motor might be corroded, 2) spark plugs may be weak, 3) seals in motor may be shot. I'll also say that I have a brand new fully charged (and checked) battery so I know thats not the issue. Also the starter seems to get hot, but that just might be a result of the cranking with nothing happening. does it sound like I'm on the right track here or is there something that one of ya'll can think of that I'm missing.

One out of three is right. You need a 1000cca battery for that engine to start and start correctly. I would be more apt to be looking at fuel pressure than any kind of seals. Seals rarely ever go out. Does your warning horn work. You should hear it each time you turn the key on. If youdon't hear it. It could be going off and trying to tell you that something is shutting the motor down or not allowing it to start.
So....Let's start with the horn. Is it working?

Thanks Mike, Yes the horn is working and there is a beep but the beep comes on when the master power switch is flipped on, not when turning key.

i agree with Mike.
Optimax engines need the 1000cca battery.

do you have a 1000cca battery?

I'm not sure i'll have to check on that. Do you think that it might not be getting enough juice from the battery? Why would it start out of the water but not in if its the battery, to much back pressure?

sorry man, i can't answer that. i'm sure Mike can.

i do know the Mercury tech that runs this area says... "Optimax engines must use the 1000cca battery"

Basically...yeah. The engine will also turn over at a higher rpm out of the water because of back pressure. When the key switch is turned on, you hear a short BEEP. After that you will not hear anything. Now that we know the horn is working. We can continue. You will need to listen for the electric fuel pump to run whenever you turn the key to the "ON" position. Without trying to crank the engine, The fuel pump should run approximately 30 seconds maybe a little longer then shut off. I that happens then we know that fuel pressure is up. If not,,,,here lyes the 1st problem. As you have three fuel pumps, One manuel, to get the fuel from the tank to the engine, #Two is a low pressure electric fuel pump and #three is a high pressure fuel pump. 2 & 3 have to work to gether to start the engine. If one fails,,,,no start. If they do what they are suppose to do. The next step is to Look at the rail that is mounted on each head. You will see a black cap covering a schrader valve,(looks like a tire valve). Press the schrader valve, You should have Fuel Under Pressure. If not, Revert to fuel pumps. IF everything is pressurized and the engine does not start. Check Spark at the plugs. If you have spark, and fuel.....and still no start... Disconnect the battery overnight. Charge the battery, reconnect, and retry the cold start procedure again. Sometimes the ECM module (brain box computer) will go into a guardian mode because the battery was not of high enough cranking amps and it will shut itself down. By disconnecting the battery it will drain it's memory and await for your command once it gets reconnected to a hot fully energized battery.
Then.........if it still does not start. Load that POS up and Take it to your nearest Authorized Mercury Dealer for repair or your local scrap metal buyer and sell it for $0.16 lb. (Just kidding, could not help myself. LOL LOL LOL)
Let us know. But definately check for the 1000cca battery and make sure it's fully charged.

+1 on the battery needing to be 1000cca and i would like to add the cables must be TIGHT and CLEAN not tighten the wing nuts with your fingers tight really TIGHT.

Have seen this several times on different motors and it has always come down to a rich condition, too much fuel.

Thanks Guys for the help. I'm going to check everything this week and if I cant figure it out off to the shop she goes. Thanks again i truly appreciate it.