Kids love to throw this torpedo at one-each-other during scuba diving lessons.
 
You will need to add some ballast (anything small and heavy will work - avoid stuff which can rust) to make sure that the torpedo's buoyancy is neutral when underwater (i.e. it shouldn't sink or float).  There is a hole at the bottom of the torpedo for that purpose. Once the ballast has been inserted, you should secure it with glue (epoxy works best).