I remember doing this...

MASS:
When an object floats, the mass of the water it displaces is equal to its own mass.
Note: The density of water is 1 g/cm3.

Mass of Water
= D x V
=1 g/cm3 x Volume displaced (1mL = 1 cm3)
= Mass of object


VOLUME:
When an object SINKS, the volume of the water it displaces is equal to its own volume. (When two objects are equal in size, this is also the volume of the object that floats)

You have M and V, substitute to find D