I guess that by an "action figure" you mean a very irregular shape. So, get hold of a couple of measuring cylinders. Fill one to the brim with water. Carefully dunk the action figure (non absorbent, I hope) in the water and catch the spillage in another cylinder. The volume of water displaced is the same as the volume of the action figure, I hope. This idea is used in the "eureka can" experiment, and I think is attributed to the Great Archimedes (circa 2000 years ago). Very useful bloke when it comes to making sure that the phrase "cast iron sinks" isn't always true.