This is one of those perennial aviation legends[0] that are still alive, like Amelia Earhart's wreck. Always another rich guy funding a search with promising results.
The terms of the lend lease agreement meant that Britain only had to pay for the equipment it kept after the war. So in some cases they were pushing aircraft of the deck of carriers into the sea so they didn’t have to pay for them.
Sorry, I wasn’t meaning to say that they were spitfires in particular that they pushed into the sea. I believe they were actually F4U Corsairs in that case.
I remember him saying that the RAF buried Spitfire engines, so it's definitely a real practice.