In this podcast, I talk about how to find the density of regular and irregular objects. A couple of extra points: 
1. The dependent variables are always the ones you are measuring. In the experiment to find the density of an irregular object, this is the volume of displaced water. I talk about the displacement, or Eureka, can but this can also be done directly in a measuring cylinder holding a known amount of water which is your starting point. If it is big enough to allow the object to be put into the cylinder, the water will rise to a new level and the difference between that and the original level will be the volume of the object.
2. The 'independent variable' is the one you change. In this experiment it is the object.
3. You measure the mass using a top-pan balance. Examiners will accept 'mass balance' or even 'scales' but not scale.