I would get another battery for the trolling motor unless you're sure
you can rope start the main motor. You don't even have to go through all
the wiring hassles and expense of the A/B switch if you hook them up to
a combiner, which distributes charging current to whichever battery
needs it, but won't let one battery discharge into the other. (Not an
isolator, which uses current even when you're not using them, I'm told.)

That way you can fish all day without fear of running your starting
battery down, then charge them both up on your way home or after you get
home and put the charger on one of them.

I can't imagine trying to rope my 90 hp Honda, although they say it can
be done.