Marine Grade means all of the plies are knot/void free and it was assembled using waterproof glue...with that said it will still rot if left to the weather.
Any exterior grade plywood should work just fine for a tool box that's going to have paint or urethane put on it.
You could make the entire box out of wood and use brass screws to fasten it together so they don't stain the wood over time like steel would.
Mike B
