referring to a body of saltwater

समुद्र

English Usage: The sea is calm today.

Hindi Usage: आज समुद्र शांत है।

letters and packages sent via postal services

डाक

English Usage: I received a letter in the mail.

Hindi Usage: मुझे डाक में एक पत्र मिला।

mail that is delivered by ship

समुद्री डाक

English Usage: Sea mail is often slower than air mail.

Hindi Usage: समुद्री डाक अक्सर हवाई डाक से धीमी होती है।