在地球日保护海洋的英文句子可以是：

1. "To protect the marine ecosystem and ensure the harmonious coexistence of man and nature."

2. "Protect the sea, love the sea and cherish the cradle of life."

3. "Ocean is the craddle of life. Save our ocean for the sake of life on the earth."

这些句子简洁明了地传达了保护海洋的重要性，适合在地球日等场合使用。