I don't use the relays for this purpose. I use a battery cutoff switch. I locate the switch by the battery and depending on what I'm putting on the switch depends on the size of the switch. Usually if I'm rigging to cutoff the front, console, anchors, depth finders, and other accessories I use one switch rated at 250A. Yes, that's overkill but I don't want a connection problem and I've found once you reach a certain size, cost is minimal to go bigger. I'll put everything except the outboard and the battery charger on that switch, then put the charger plug next to the switch so when you grab the charger plug it reminds you to turn the power off. You should also have something like this on the trolling motor batteries to keep a parasitic draw from pulling those batteries down. Also, allows you to charge without unplugging when you turn it off. Before long, it happens when you're removing or connecting the tie down straps as part of the loading/unloading ritual.
John