I wondered about those things. There was a beach in the Caribbean, that the old sailing vessels anchored in the shallow water, that a hotel owner told me about. There was ballast everywhere. He said the worms in the warm water would eat the wood and some sank. He used hotel staff to walk around in bare feet and advise when they felt a cannon. He retrieved a number of iron cannons plus one small deck bronze cannon. There were also lots of those spikes, which is why he stopped that discovery method.