Danish archaeologists investigated the Naval bases of classical Athens. Much of what remains is under water in the harbors of Mounichia and Zea, parts of the port of Piraeus. (Sea levels have risen a lot in the Mediterranean because of something called "isostatic rebound": the glaciers of the last Ice Age pushed the northern Europe down, tilting the whole continent and lifting up the southern edge; since the ice melted the north has been slowly rising, and the south sinking. There are also local effects due to earthquakes etc.)
The Danes think the first slipways at Mounichia were built around 490 BCE, when Themistocles persuaded his fellow Athenians to invest the windfall of silver from the Laurion mines in a powerful fleet. The ship houses were built up gradually over the second half of the century.
I would love to read more about this, but in the archaic way of classical archaeologists these Danes are publishing their findings in fat books that cost $100 each, and their web site has next to nothing in the way of scholarly information. Grrrr. When I run the world, all archaeological publishing will be done on the web.