askchapo

Mediterranean to start with. It's huge and relatively calm to sail on, so lotsa trade, meaning lots of practice (and profit) in improving those trading ships. Rome took this to new (horrific) heights; mass (slave) produced, standardised pottery from North Africa can be found across the entire empire, from Jerusalem to London. Rome's power (and wealth) was built on this slave labour (both in factories and in villas and in boats).

Rome's collapse slowed the marketization and decreased the scale, but on the whole by this point, such methods of transport and trade had reached the northern coast of Europe, which has a similarly large, but less calm sea. Traders here needed more navigatory techniques, and of course traders going all the way around Iberia, the sea route connecting these two seas, requires naval expertise.

Europe's polities are tiny and constantly fighting, in need of cash to pay armies (increasingly, mercenary armies). Merchants are hence supported, sponsered, etc. From here, see my points about the clock and navigatory technology above.