The confusing world of batteries half art and half science. On our Maramu we use Trojan 6V golf cart batteries. They are readily available in Brisbane. Not sure if it is worth changing all the cables from 12V though and you would obviously need to check the case sizes.
I've had maintenace free before but if the batteries are worked hard then the stickers have to be removed to get at the caps to check the fluid level and top up with distilled water as necessary, so not much point. Personally I think the cost of AGM and Gel is only worth it if you need to charge very fast, faster than a wet battery can cope with, or you think your boat might end up upside down one day!.
You also have choices whether you want calcium, silver calcium etc......
John, Maramu #91 Popeye, Port Douglas