In deep water I always dumped all metal cans overboard. Punched extra holes in aluminum drink cans to make sure they sunk. Even if it took a few years to corrode away (which I don't believe is true) they are sitting on the bottom, several thousand feet deep so why should anyone care.

Most of that plastic garbage washed up on beaches is not from small boats. It is from commercial craft that dump at sea and from improper dumping of trash from shore. I have seen large fishing vessels with their stern net slides loaded with trash. I was told that it was too much trouble to take it off the boat while in port and that when they get to sea they will dump it.

I never dumped plastic in the water in any form. Melting it into a hockey puck does not shorten it's life and it will float. Burning it is a better solution and that is what happens on most islands away from the U.S. If it won't biodegrade eventually it should not go in the sea.