The sea is down.
The English translation seems certifiably bonkers. If someone said this to me in English, I would have no idea what they meant, and I’m a native speaker. Suggestions: “the tide is low”, or “the sea is low”. The sea is “down” is nonsense.
The sea is down.
Thank you for this comment. There are several translations that would be so much better if they were tweaked a bit.
I did an internet search, and I found La mer est basse in some different contexts, so it sounds like the English may be more bonkers than the French.
Indeed, “La mer est haute/basse.” is fine in French.
You may also hear: “marée haute” (high tide) or “marée basse” (low tide).