i recently bought a 2004 prowler 26 496ho and am experiencing a draw on the batteries and overnight they are completely drained. I was wondering if anyone had some insight on troubleshooting this situation. I have noticed with the key off and just the batteries on the ignition wires on the starter are warm when the motor hasnt even been started or key even turned on. I am also experiencing problems where it sounds like the starter is getting low voltage when cranking. I have 2 brand new batteries and it looks like the previous owner installed an ignition switch ( not mercruiser ) if that matters. Not sure where to go from here any help would be appreciated.

A test light and multi meter will go a long way here . You need to start probing around.

check with merc i read on the net that some 496 pcm had diode problems on the ignition circuits .

Is the aftermarket starter switch a marine unit? If not, I would not be surprised if it has corroded and is causing some or all of your problem.

Check the major fuse breakers under the back seat to make sure there is no fault there.
Also, make sure to check the battery switch. They can go bad.

Thanks guys will check some of this stuff and start probing around tomorrow.

First turn everything off like you would leave the boat overnight. Disconnect the negative battery cable, connect the cable to the battery with a test light (it should light up if there is a draw).

Now start disconnecting things, pulling fuses, one thing at a time until the light goes out - thats the circuit that has the issue

I agree. Place the test light between the disconnected neg cable and neg battery terminal.

This will work unless the load is to great - and blows the bulb in the test light. I use an ammeter when tracking down parasitic drains. Most newer DVOMs have a fused ammeter on board.