'Wondrous' means so good, so beautiful, or so strange that it amazes you. It means almost exactly what 'wonderful' means, but it does not sound like it. 'Wonderful' is a warm everyday word you can use about a meal or a weekend. 'Wondrous' is a raised, poetic word: you meet it in poems, hymns, old translations, fantasy stories, and film titles. Using it about small things is a joke — 'this wondrous sandwich' is funny on purpose. Used straight, it belongs to big things: a wondrous sight, a wondrous machine, the wondrous variety of life. There is also an old use as an adverb, meaning 'amazingly' — 'wondrous strange' in Shakespeare — and you will still see it in poetry, though nobody says it today. The word also leans a little toward strangeness in a way 'wonderful' does not: something wondrous may be beautiful and also hard to understand, even a bit frightening in its size. That shade is why it works so well for the sea, the stars, and gods.