Oh, when she was sad, she cast herself down into such a pure depth, (10)
Jane's sadness is no joke. The description of Jane, "cast[ing] herself down" echoes the action of someone throwing themselves off a cliff into the "pure depth" of an abyss. Bummer. It is as if Jane participates in the sadness—she goes into it. She isn't just sitting there being sad, she throws herself into it.
Even a father could not find her; (11)
Jane's sadness is so profound, she's beyond anyone's help. No one has the power to cheer her up. By using the phrase, "could not find her," Roethke makes being desperately sad kind of like being lost. Why do you think Roethke chose "a father" to describe just how unsavable Jane was? Would it have been different if it had been a mother, or sister, or brother who was unable to find Jane?
Scraping her cheek against the straw;Stirring the clearest water. (12-13)
Ever had straw in your shirt (or, worse yet, your pants)? Pretty uncomfortable. It's scratchy, irritating stuff. Rubbing your cheek against it isn't going to feel very good at all.
This line gives us a physical sense of Jane's painful psychological state. Why would someone stir up nice clear water? Well, maybe they don't want to see themselves reflected in that crystal clear water—kind of like when someone can't look at themselves in the mirror. Stir up the water in a pond or lake, and all the mucky stuff that has settled down to the bottom comes up to the top and makes the water murky, the way sadness can cloud the way we see things.