I am of no help on a replacement alternator, however, many of the older alternators needed to get above 1500 RPMs to excite the alternator field and start charging. One of the first time I took my boat out I had previously only run it at idle in the driveway, then idles through a no wake zone and idled across the lake. Didn't want to start once we were ready to ski. Easy to check. Might also get your battery tested to eliminate it as a possible source.
CUSTOM WHEEL & TIRE PACKAGES
$125 LED LIGHT PACKAGE - FREE SHIPPING
REBUILDING HOLLEY MARINE CARBURETORS - $249 DELIVERED
REBUILT COMPLETE MARINE HOLLEY 4160 CARBS - $425 DELIVERED
Email - email@example.com