I first thought

Come all you seamen bold = A

And draw near = B

It's of an admiral's fame, = C

O brave Benbow was his name, =C

How he fought all on the main = A

which would give us

ABBABCCABB

I prefer

Come all you seamen bold,
And draw near, and draw near,         = A

Come all you seamen bold
And draw near.                        = A (near as dammit)

It's of an admiral's fame,
O brave Benbow was his name,        = B

How he fought all on the main,
You shall hear, you shall hear.        = A

or

AABA

much more familiar and also works melodically as well as verbally.