The traditional farewell (or toast) of a mariner is wishing a friend "fair winds and following seas." The full version is "Fair winds and following seas and long may your big jib draw!" "We bid shipmates farewell with this naval blessing because it represents the ideal underway conditions for which Sailors yearn."
The phrase may also be considered a Naval blessing as the person, group, or thing to which it is said departs on a voyage in life. It is often used at a "beginning" ceremony such as a commissioning ceremony of a ship or people, as well as in retirement, change of command, or farewell ceremonies. It is also said for a departed mariner at a funeral.
'Following sea' refers to a sea swell that doesn't overtake the ship. It is very difficult to control any boat or ship when the swell overtakes it and shoves it forward, causing the boat to skew broadside-on to the wind and swell. Boats often capsize under such conditions.