अन्वयः

तदा then, कपि: vanara, उपरिष्टात् the top, शरीरेण with his body, सागरे in the ocean, अवगाढया reflected, छायया च shadow also, मारुताविष्टा swept by the wind, नौरिव like the ship, आसीत् appeared.

M N Dutt

As he coursed above, on account of the reflection cast by his body (on the boson of the deep), the monkey* by his progress, looked link a bark swayed by the breeze. *Under water.

Summary

When the vanara was flying in the sky, his shadow reflected in the ocean looked like a ship driven by the wind and sailing on the sea below (as if the lower part of his body was under water and upper part occupied the sky)

पदच्छेदः

उपरिष्टाच्छरीरेण	उपरिष्टात् (अव्ययः)–शरीर (३.१)
छायया	छाया (३.१)
चावगाढया	च (अव्ययः)–अवगाढ (√अव-गाह् + क्त, ३.१)
सागरे	सागर (७.१)
मारुताविष्टा	मारुत–आविष्ट (√आ-विश् + क्त, १.१)
नौर्	नौ (१.१)
इवासीत्	इव (अव्ययः)–आसीत् (√अस् लङ् प्र.पु. एक.)
तदा	तदा (अव्ययः)
कपिः	कपि (१.१)

छन्दः

अनुष्टुप् [८]

छन्दोविश्लेषणम्

१	२	३	४	५	६	७	८
उ	प	रि	ष्टा	च्छ	री	रे	ण
छा	य	या	चा	व	गा	ढ	या
सा	ग	रे	मा	रु	ता	वि	ष्टा
नौ	रि	वा	सी	त्त	दा	क	पिः