Place the marzipan in a large mixing bowl with the powdered sugar. Use your hands to knead the mixture until the powdered sugar is incorporated. The marzipan will be stiff but should be malleable and able to be rolled into small balls without crumbling apart. If necessary, add the water and more if necessary, a few drops at a time.
Break off pieces of the marzipan and roll them into 3/4 inch sized balls.
Place the cocoa powder in a small bowl and add the marzipan balls in a few at a time, tossing them in the chocolate until coated.
Place a sieve over the bowl and shake the chocolate-covered marzipan balls in it to release the excess chocolate.
Store in an airtight container. (If you used the traditional egg white when making the homemade marzipan, keep them stored in the fridge for up to two weeks. If you used corn syrup you don't need to keep them in the fridge and they will last even longer. Note, if made with corn syrup the texture won't be as tender.)