海运 (hǎi yùn) means 'to ship by sea' or 'sea freight' — it specifically refers to transporting cargo via ships. In contrast, 航海 (háng hǎi) means 'to sail' or 'to navigate,' focusing on the act of traveling the seas (e.g., a voyage). While both involve the sea, 海运 is commercial shipping, and 航海 is about sailing or navigation.