The third mate is the Watchstander.

when all to rest, weary to unload,
eager to embark to station of quietude
he is at attention, full alert.

he is the Safety Officer.

your care to him entrusted.

you can read this all beneath his banner.

he to duty called,
sharp the alertness,
his whistle and his words
every ready, always, always,
a poem at the ready.

how else does a third mate make you safe?