It is your shadow in the deep sea.
Shinkai de kimi no kage ga yureru.
deep sea; depths of the sea; ocean depths
indicates location of action; at; in; indicates time of action; indicates means of action; cause of effect; by; and then; so; indicates continuing action; indicates certainty, emphasis, etc.
you; buddy; pal; monarch; ruler; sovereign; (one's) master
indicates possessive; nominalizes verbs and adjectives; substitutes for "ga" in subordinate phrases; (at sentence-end, falling tone) indicates a confident conclusion; (at sentence-end) indicates emotional emphasis; (at sentence-end, rising tone) indicates question
shadow; silhouette; figure; shape; reflection; image; ominous sign; light (stars, moon); trace; shadow (of one's former self)
indicates sentence subject (occasionally object); indicates possessive (esp. in literary expressions); but; however; still; and
to shake; to sway