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I saw a fairly interesting talk on this (abstract, for the curious). The basic conclusion is that they relied heavily on expert knowledge, and knowledge of waterways and wind and current patterns was something highly valued in the ancient world. There were charts of a sort, but comparatively rare. "Periploi" are more common and were basically verbal descriptions of sea routes (ie, two days to x, then three to y). But these never overtook the vital importance of having a strong internal grip on the hydrology and personal communication with other navigators.