The problem with your hull is common to DB bottoms, it's starts with how they're made and the solution for a permanent fix is a little more involved than just filling the crack. The common method of building DB bottoms is to use multiple layers of 24 oz roving with no mat between them, this works for a while, but as the hull gets stressed over time (used in shallow water and flexing over or banging into rocks) these layers of roving tend to separate. layers of roving need to have mat used between them for a better bond and this step gets left out to make them lighter. As you start to grind out the damaged area you will find the area will be larger than you thought, it won't be confined to just what you can see from the surface. Water will also have filled this void, so you'll need to grind it out and dry it completely. To rebuild it you can use polyester or epoxy resin, epoxy is stronger, sticks better and will cost more. Polyester is what your boat is made from and it will work almost as well for this type of repair. For a repair with polyester resin you will need to use mat and roving, the mat needs to go down first and then used between each layer of roving, you don't need to use mat with epoxy just cloth and/or roving will work. Fill the area back up so it's level with the surround floor and then sand it smooth. It's not hard to do, shouldn't take very long and supplies aren't very hard to find, the only tool you need is a small grinder.