None, since there is no such thing as a millaliter. Furthermore, marbles come in all sorts of sizes.
Not always. The density of water is 1 gram/mL at around 4 degrees Celsius. As temperature rises, kinetic effects kick in causing the volume per unit mass of water to increase, thus decreasing the density of water. Interestingly enough, the density of ice (frozen water) is also less than liquid water. The formation of ordered crystal lattices occupying relatively large amounts of space causes the rise in volume and an overall drop in density.