I had the same experience with my 1989 vintage merc/mariner 75. It started
with a new battery.
All I could think was that they changed the way they make
batteries.

That is exactly how mine did it. New battery and all the trouble started.
Every time I put the old battery back in it was fine but finally it just died
from old age. It lasted 4 years.

I had that same thought as you but from what I can see, refillable flooded cell
batteries are no different. Outside of adding some LED lighting, nothing on my
boat changed but the battery. I thought the LED's could have been suspect but
I took everything out of the system but the motor and it didnt change.
It almost has to be something with the battery. Right now I'm trying to find
out from an expert if a bigger CCA battery or a smaller CCA battery could make
a difference since it is the regulator in the system.

Thanks for the info.