Batteries and salt water?

I just bought 2 kayaks off PFF and they both have trolling motors and full size batteries. They are strapped to the top/back of the kayak in plastic battery boxes. Being new to kayak fishing im just wondering how careful i have to be about these batteries staying completely dry? They are marine Exxide batteries but the box is obviously not in an air/water tight space so water will be a factor.
Just wanted to hear thoughts or safety tips or lessons learned......