Sinking into the sea means:

The stone sank to the bottom of the sea. Metaphor is never heard from again.

[Pinyin]

Hitchin dahai

[source]

Bird Bao Guo's "Luo Li Lang makes a scene in Suoguo Temple": "No one knows when you go out; Like a stone sinking into the sea; Iron falling Jiang Tao; Pennisetum has no roots; Broken kite. "

[example]

It seems that Wang Ming hasn't received any news from him since he posted the manuscript.