[The_Victor]

The bottom of the jars fit into racks which had holes in them. Much more secure than a flat bottomed jar on a pitching ship.

Makes sense. And I guess the wooden racks don't usually survive underwater, which is why I've not seen one when archaeologists are excavating a bronze age shipwreck?

[buffaloguy]

Wood tends to go very rapidly underwater although there may have been a wreck or two where the racks still survived. It would have to be in an area in which the oxygen would been depleted.