The "Green board" refers to the "Christmas Tree" or Hull Opening
Status Panel found on the old diesel submarines (see below.) This
panel used red and green lamps to show whether hull openings
were open or closed. Red meant open, green meant closed.
Hence the name "Christmas Tree". If all lights were green, meaning all
major hull openings were closed, it was safe to submerge the ship.

When this occurred the diving officer of the watch announced a
"Greenboard"to the officer of the deck. And he would order the ship
to be submerged, by passing the words "Diving officer submerge the ship."

The diving officer, unless otherwise ordered, would order a standard
down bubble, Pass the words "Dive! Dive!" on the 1MC, and sound the diving
alarm klaxon twice.

Since a "Green board" inferred a safe ship, and a safe journey, the term is
used to wish someone a safe journey through life.