I have a Crest 2003 Family Fish Pontoon with a 75 HP Mercury engine.
Recently, the 20 Amp Starter/Accessory/Trim fuse began to blow. I took the boat into the dealer where it was purchased and they spent ~$400 to tell me that the engine wiring was A-OK, but they could not fix the problem.
The hardest part is that the fuse will not blow unless the boat is in the water. I have tried to duplicate everything leading up to the blown fuse while running the engine with a water hose connected and have not been able to get it to fail. Then I put the boat in the water and 10 to 90 seconds later (after putting it in gear), it blows. I lose the instruments, trim, but the engine continues to run fine. If I shut down, I can not restart without replacing the fuse.
I tried on land to ground the pontoons to see if that was it. Then I sprayed water on the wiring from the engine to the controller (rests on top of a pontoon). I removed the gas level transmitter. I removed the live well pump. I tried everything I could imagine.
I know this fuse enables power to the starter relay, and the up and down trim relays. They were checked out by the shop and found to be OK. I also measured the running voltage on the rectifier at 14.8 amps with the engine throttled up. The boat is simply instrumented: Tach, voltmeter, fuel gauge and that's it.
Sure could use some sound advice.
At the key switch, disconnect all wires connected to the A or I terminal. These should be purple. This will cut power to your gauges and anything else connected to the key.
If your key switch is incorporated with your shifter look for the purple wire coming out of the control. The motor will start and shut down without a problem.
If the fuse still blows, look for a bad spot in the wiring between the engine and the key. If it doesn't, then the problem is in one of the accessories connected to the key switch. It could be something as crazy as your stereo or tachometer.
Hope this helps