It's so small it is probably gelcoat but that can still allow water to soak into the mat over time. I would remove the bolt, grind the crack open a little, drill a small hole at the far end to relieve any stress and fill it with thickened epoxy resin overlapping the margins a bit. Fair it in, then paint over it. Unless it turns out to go through the mat and would therefore be structural, it will probably not need any reinforcing with additional glass. If it appears to go through the mat when you open it up, ask again. A really good resource with tons of solid DIY information and tech tips is the Gougeon Bros. (West Systems Epoxy) website: <http://www.westsystem.com/ss/>
It's what I would do if I were worried about it, which I honestly would not be, it's not like a boat hull that is completely immersed in water. Unless you live in Seattle that is.