As part of a "double dare" I have to start posting more frequently, so here is a quick post to get things rolling again.
The Midrash Tanchumah on Parashas Mikeitz states the following:
"And from where do the rains descend? Rabbi Eliezer says: All the world drinks from the waters of the Ocean. Rabbi Yehoshua said to him: But are not the waters of the Ocean salty? He said to him: They are sweetened by the clouds of the sky (rakia); for Rabbi Shimon ben Lakish said: Why are they [the heavens] called shechokim? Because they grind [shochakin] the waters and sweeten them, and after that, they descend."
There are a few interesting things to note in this passage:
1) Chazal seems to have known that the water in the Oceans rises up in vapor to form clouds, which in the process filter out the salt, and then the clouds blow onto land to give rain (I don't know if this knowledge was prevalent then or not, but it seems to me to be pretty advanced knowledge for that time).
2) The rakia includes the sky which contains the clouds, meaning that it cannot refer only to the "dome" or to space, etc. but is more inclusive.
3) Rabbi Eliezer says that ALL the world drinks from the Ocean [ok'yanos]. What about underwater springs, rivers, lakes, wells, etc? Some possible answers I can think of off the top of my head (i.e. may be totally wrong) are that for rivers, they flow due to the rain, for lakes, they formed from rain, or from when the land was gathered during creation, and hence from the same body of water as the Oceans (before they complained and became salty), and underwater springs and wells could be formed from absorbed rain.