Has anyone used a 36V Golf Cart Battery Charger to charge a three battery setup?
Golf Carts use 6 each 6V batteries wired in series to get 36V (some use 48V) and the chargers put out 36V usually at about 5 Amps.
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They are considerably less expensive than a 3 bank marine charger. I am considering this one:

Stanley - Golf Cart & Vehicle Battery Charger - 6V to 48V - Sam's Club

There are on-board chargers also available albeit they do not appear to be waterproof. Like this one:

NEW 36 Volt Battery Charger Golf Cart 36V Charger For Ez Go Club Car DS EZgo TXT

On the second one the output plug would have to be removed but would be an easy task. The first one I linked comes with an alligator clip adapter.