I took the boat in to the dealer when the second battery failed. He did a complete check on the boat electrical system and did not find anything that was causing the problem.

I bought this boat in August so it is still under warranty. So I will pull boat from the slip and take it back to the dealer in June to check the electrical system again. This 3rd battery is starting to have problems cranking the motor.

Health problems keep me from doing it sooner.

No I asked about that and my mechanic said it was 18 month warranty.

I had a bad charger, did not know it, killed two batteries dead.

They checked the charger when they checked my electrical system. They installed the charger on the boat when I bought it plus the trolling motor with it's two batteries and charger.

I hope the trolling motor batteries are still good. I have been unable to use the trolling motor for months due to health problems.

Tommy,
I installed a positive disconnect switch on a boat I once owned and my battery problems quit.
Might something as simple as a radio. They look like they are not on but the internals are still powered.

I can understand how this might happen if you have equipment in the circuit that stays on such as radios. But how does a constant drain on the battery cause the battery to develop an internal short?

Also why wouldn't the trickle charger keep the battery charged if something like a radio was draining it?

I won't allow a radio on a boat I own. The only thing on the starter battery is motor, the nav lights, courtesy lights, the hour meter, the fish finder and the pump for the live wells.

There might be something in the mercury outboard motor that stays on all the time but the other items are controlled by switches which remove the current from them when they are off. You can hear or see them when they are on. The fish finder is removed from the boat when I am not using it.

As stated I am going to take the back back to the dealer to have the electrical system checked again, since the 3rd battery in less than a year is starting to fail. They did not find anything the first time but maybe they will when they check it again.

Hopefully by now they will have a new shipment of interstate batteries and the new batteries will not have this problem.

Back to my original post. ..I trickle charged my battery and so far my starter/cranking is working good.

Disconnect the negative terminal after it's charged,
if a connected item is draining then your battery should
show charged for an allotted amount of time.
If the battery drains down disconnected, then it's the battery.
While it's disconnected you could go in the evening and
touch a negative wire one at a time and see if you get a spark,
if you do, bad switch or short somewhere in the wiring or component.
Or you could put a voltmeter in line one wire at a time and see if
you get a connection. If you show volts, bad switch, fuse panel short, wiring, item component.

The dealer thought the problem might be in the boat wiring when the second battery failed. So I took the boat in for them to check.

The dealer had the boat in their shop for a week and did a complete check on the boat's electrical system when they replaced the second battery. They stated there wasn't anything wrong with the boat's electrical system. They stated the problem was shorted plates in the battery, which they replaced.

If you like this battery, then continue to use it. I will not buy it again and if it is on a boat I buy, I will require the dealer to replace it with a different brand before I buy the boat.