What do you think happened? Seems to small to be a Treasury shipment to pay Legionnaires. Maybe shipping coins to a remote outpost on the Med or further away? Maybe somebody transporting his own personal wealth?
Kaboomed into a rock, most likely. Generally, the risk is greater the nearer one is to land.
It would be nice to see what other cargo was aboard. A pile of coins that big suggests it was a big ‘un, maybe one of the Roman grain haulers. THAT would be excellent.
Lionel Casson wrote that travelers in the know back then, if on a long Mediterranean journey, would pay for their passage aboard one of those. They were much more roomy, offered more privacy, and were much safer as well.