Right then. The Wiki original version has a verse

He is the King of Reef, I'll declare, I'll declare,
He is the King of Reef, I'll declare,
He is the King of Reef,
Of a Robber and o' Thief,
To rest void of Relief when he's near.

So what was reef in this context - online dictionary gives
"verb (used with object)
2.to shorten (sail) by tying in one or more reefs.
3.to reduce the length of (a topmast, a bowsprit, etc.), as by lowering, sliding inboard, or the like.
4.to pull (old oakum) out of seams, as with a rave hook (often followed by out)."
and the Burns glossary
"Reef, Rief
Plenty"

None of those seem to fit.