Or did they see landmasses separated by ocean to be entirely unconnected to each other and for oceans to be water "all the way down" (whatever that may be)? Or did they perhaps even see landmasses as floating on a large single body of water?
"Ancient Greeks" spans a large amount of time so you may want to specify which period you are interested in. However, in the book Mythology by Edith Hamilton that I'm reading now the author mentioned that in early Greek mythology the Titan Oceanus is the physical embodiment of the ocean, being a river that spans the entire length of the Earth.