Hot wires when the battery is connected -- throws a spark??

Just bought a pontoon off a neighbor, and appears to be about 1974. Motor is a 1974 50HP Johnson. Got it home, parked in the driveway, and when I hooked up the wires to the battery, it through a spark at the connectors. All lights and accessories have been disconnected and stripped from the boat -- the only thing that is still connected is the motor. Started up just fine for a few seconds, but the wires still spark when I hook the battery up. Yes... I know... the positive and negative is in the right place. Left it connected overnight, and ran the battery completely down -- when I tried to charge the battery, my solid state charger kicked out a F01 error -- bad cell, replace battery. Anyone have any suggestions as to how I should proceed with troubleshooting this problem, and what exactly would be a logical progression through this troubleshooting? Any advise would be greatly appreciated.

Try hooking an amp meter in series with the positive lead and bat.terminal measure what current it is drawing withevery thing switched off there shouldent be any current drawn if there is disconnect different items one at a time (like radio ,ignition, tilt etc.) till meter shows no current flow thet will tell you where the short is

OK -- took me a while, but I finally got around to the problem with the motor. As I said -- the only thing I have hooked to the battery is the motor -- no lights, not anything. Today, I decided to take the controls apart to see if it could possibly be a bad key switch. Now, the interesting thing, when I was tapping the positive cable on the battery post, I heard a ringing inside the flywheel. When I turned the flywheel slightly by hand, and tapped the post again, the stator appears to be wanting to turn the motor -- does this sound like a bad regulator? If you tap the cable on the post, the current is feeding into the stator and the stator appears to want to behave like a really weak motor and wants to turn the motor. Kinda sounds like a bad regulator to me, but what do I know. Anyone know the proper method for testing the regulator with a multimeter? Could it possibly be anything else?

Thanks so much for the info!! I don't think you have to worry about me not disconnecting the battery -- in its current state, if I leave the battery connected, it will drain the battery completely -- ruined a good battery and now have a brand new one. As long as I see current at the positive post of the battery when I try to connect the cable, I'm not leaving it connected!