Why do they sink decommissioned ships?

Answer 1

From the late 1880's up until the 1960's they were sunk to TEST weapons and train personnel in the operating of those same weapons; training purposes. Today, Environmental Groups have restricted the scrapping/re-cycling of old warships to the point that they are too expensive to scrap out (re-cycle). They are cheaper to just sink. Consequently, decommissioned ships are sunk for target practice, as well as to test out some new ordnance.