If you don't already have a charging circuit on your outboard, but you're lucky enough that it already has generator coils installed, you might consider adding the missing rectifier/regulator to charge your battery bank.
That was the case recently with my 2004 8HP Yamaha 4-cycle. It wasn't hard to find the part (used, even) and install. Now my battery can recharge with 2-5amps whenever we're motoring. It might not meet all of our future on-water charging needs but for only $71, it's worth a try. Time will tell.
I published the full story today in Outboard motor charging solution
on my blog.