Shorts out in the heat

Yes, that is exactly it. I installed the new switch to support the second battery to run the stereo and amp. Folks on this forum recommended this path to me and it worked great last summer.

What exactly is the black electronic box on the back of the engine block and what about the engine getting hot would cause it to trip?

It might not be grounded.. When I installed the new switch, I removed the red and black wire in the old switch and put them on the same places on the new switch. Then I attached the second red wire from the new battery to the switch. I'm sorry that once again, a little knowledge is dangerous in my case. I don't know where the black lead ll to the switch comes from I assumed it is the ground. I can send a wiring diagram later but it would the be the standard as I didn't change the wiring, just move the leads to the new switch.

The switch is the battery cut off switch. It's a dial. Red and black. Mounted to the bottom back right of the engine block.. The red from each battery is attached at the 1 and 2 posts. And it's grounded. When I turned off the ignition on the boat, all power was lost-blower, stereo. I opened the engine cover and saw the black box (fuses, switches?) with the red reset button. I could push this button, but only after a period of time.. 10 minutes or so, did power come back to the accessories and ignition.

I installed a stereo, second battery and new battery switch last summer. The amp is connected directly to the second battery as this forum suggested last summer. Took it out for the first real time today. After running for a couple of minutes at 15 mph I turned it off to swim. When I turned it off, all power was lost. There's the black box on the back of the engine that has a red reset button. After the boat cooled sufficiently I could push this reset button to fix the short. Any ideas what is happening? Thanks in advance.