There couldn't have been rocks underneath as Peter began to sink. John was the one who talked smack about Peter.

For historical accounts from that time, 5 decades after is rather close. Most records we have about history from that point in time are written centuries later. Generally copies of copies, etc. When mark wrote it though, there'd be several other guys who would have been there who could have said "actually this didn't happen", by this point they were spreading all over the world, but they already accepted Mark's gospel.

Also worth noting that the 5 decades date primarily comes from the presumption that Jesus couldn't have told the future in the Olivet discourse. Which if Christianity is true, the account could very well have come earlier.