well one of about 3 reasons. 1, starter is nfg
                                              2, wire to start is bad or going bad
                                              3, you have a ground prob in the boat somewhere???

 start with the starter, it should be the prob, good luck.

 Does it spark (slightly) when you first connect it ?  That can be caused by a lot of things.  Memory wire for a stereo, amplifiers, etc, etc.  Most of the time its not an issue, just an initial draw when things are first connected.

 Or is it a BIG spark and the wire gets hot with nothing turned on ?

Do not reconnect the positive terminal.  A dead short can result in your battery exploding.  Sounds like you have a dead short in the starter someplace.  Is the solenoid integral with the starter?  Remove the starter as an assembly, using a continuity tester or ohm meter check if the positive connection on the solenoid and starter housing makes a complete circuit.  Remove the solenoid and do the same where the connection is on the starter where power is supplied to the brushes and the starter housing.  What you are doing is determining if the short is in the solenoid itself or the starter.  that will tell you the next parts you need to change or repair.

well check to see if your wires were crossed, somethin aint right. it should not have anything to do with your coil. its between your batt and the starter. do you have a fuse block? try this  pull alll the fuses then put them back in one at a time checking each time .

Ok found out it was the ignition switch and a bad battery cable now it fired right up 2 Gage cables . thanks eveybody for the help. How do I post pictures ? Thanks