Technology post-WWII progressed very rapidly, rendering most of the equipment used to fight the war obsolete. The Tank Destroyer Branch, being flawed in the face of developing tank doctrine after the war, was shut down immediately after it ended. This left thousands of vehicles that really didn't have any use anymore. Light tanks that were sometimes pre-war were now woefully inadequate. The rapid pace of the development of jet aircraft rendered propeller-driven planes obsolete. The development of jets made most naval and land antiaircraft weapons (hydraulically traversed machine guns and cannons) and hence their ammunition ineffective; modern antiaircraft defense focused on missiles.

Much of this equipment that was discarded, like planes, tanks, weapons, and some types of ammunition, were reclaimed for the metal and other materials that was in them (to use them in the development of new objects) But, as you have noted, some of these vehicles and weapons were in fact dumped into the ocean. Small arms ammunition such as handgun bullets can't simply be thrown into a smelter; individual bullets need to be disassembled in some way and the powder and primer taken out. This is a very tedious process, and if you are dealing with hundreds of millions (often billions) of rounds of ammunition whose weapons systems are not in use anymore, dumping the obsolete rounds into the sea instead of wasting thousands of man-hours disassembling them by hand is a better choice. The US produced over a billion rounds of artillery, 100 million grenades and mines, and our small-arms plants had a capacity to produce 20 billion rounds per year.

Sources:

History of the Ammunition Industrial Base; Joint Munitions Command

http://www.jmc.army.mil/Docs/History/Ammunition%20Industrial%20Base%20v2%20-%202010%20update.pdf