Tom Lewis wrote the tune and the words of the verses.  The origins of the chorus are less certain, but can be regarded as traditional.  Tom is quoted as saying that he found it as a verse cited as "traditional" in the book "Send Down a Dove" by Charles MacHardy which was first published in 1968.  Mudcat sources reveal a similar song sung by a 20th century Australian seaman (with a similar tune) and some interesting slightly older Irish variants:

Sailor's Prayer                       (Tom Lewis/Traditional)

This dirty town has been my home
since last time I was sailing,
But I'll not stay another day,
I'd sooner go out whaling,

Oh lord above, send down a dove
with wings as sharp as razors,
To cut the throats of them there blokes
what sells bad beer to sailors!

Paid off m' 'score' and then ashore,
m' money soon was flying,
With Judy Lee all on my knee
and in my ear a lying.

With m' new-found friends, m' money spends,
just as fast as winking,
But when I make to clear the slate
the landlord says: "Keep drinking!"

With m' payoff gone, and clothes in pawn
and Judy set for leaving,
Six months' of pay's gone in three days
but Judy isn't grieving.

When the crimp comes round I'll take his pound
and his hand I'll be shaking,
Tomorrow morn' sail for The Horn
just as the dawn is breaking.

Well it's one last trip from port I'll ship
but next time back I'm swearing,
I'll settle down in my home town,
no more I'll go seafaring.