Can anyone think of any problem with adding another diode bank to my
90hp Merc 2cycle so that it could charge another different battery?
I assume that the diodes are nothing more than a bridge rectifier
setup so adding another set of diodes would just provide another
parallel load to the alternator stator coil. I assume that since the
alternator is by and large a current source anyway that the only
effect of adding another bridge rectifier is that some of the charging
current that normally goes to the starting motor would now get
directed to this other battery.

I know that there's other ways of doing this, battery isolators etc,
but it would seem that if I have space under there and have the
diodes that this would be a reasonable way to go about it.

Any reason why not? I'm no longer under warranty. Worst case if
there were excessive loading I could burn the stator coil of the
alternator if the current were not limited.

tks
Don