Probably worth putting a coat or two of varnish on the wood beforehand too, Alison. A4 or 316 stainless steel “penny” washers are great for applications like this, and I try to use nyloc nuts throughout my boat; there’s never really a good time to have a nut come loose, and if it does it will more often than not find its way under the floorboards, overboard or into some long grass.