Any difference here?  I put a Duracell 12v auto battery in and it lasted 1 year 3 months.  Yuck!  I wouldn't think there's a difference.  Thoughts...?

There are some fairly major differences

 

https://knowhow.napaonline.com/marine-battery-vs-car-battery-what-are-the-differences/

https://www.centurybatteries.com.au/products/marine/car-batteries-vs-marine-batteries

 

You are lucky  if a car battery  last a year as a starting battery.

They are just not designed to take the abuse.

If it was just for cranking the outboard...

I think a car battery would be OK ...the outboard alternator would keep it charging.

The big problem I see is ...if you can drain all the power from a regular battery it basically dead

While you can just recharge a  marine battery. Most people will buy a marine battery for cranking because they run a bunch of other electronics off it too.

I had a boat in my late teens. I was told then a marine battery is physically more robust to handle the vibrations and hard knocks. 
 

I believed them then. 

Car battery won't take the constant charging and discharging if used without an alternator to Maintain it's charge. Like already said if just starting an outboard and then being charged by it, your pretty good, but on a troller it won't last too long.

On 9/1/2021 at 1:52 PM, Bartableman7 said:

Thoughts...?

Unless you're in some emergency situation, get a marine battery for your boat.  They're different in numerous ways, and last much longer than your car battery.  In a pinch, I've borrowed the battery from the truck to get through a tournament when I had a bad cranking battery.  I think I probably only started the boat three times that day.