Help needed about Stevenson's "The Merry Men"

‘Na, na,’ he said, ‘no as bad as that. I had been drinking, though. And to tell ye the God’s truth, it’s a thing I cannae mend. There’s nae soberer man than me in my ordnar; but when I hear the wind blaw in my lug, it’s my belief that I gang gyte.’
‘You are a religious man,’ I replied, ‘and this is sin’.
‘Ou,’ he returned, ‘if it wasnae sin, I dinnae ken that I would care for’t. Ye see, man, it’s defiance. There’s a sair spang o’ the auld sin o’ the warld in you sea; it’s an unchristian business at the best o’t; an’ whiles when it gets up, an’ the wind skreights — the wind an’ her are a kind of sib, I’m thinkin’ — an’ thae Merry Men, the daft callants, blawin’ and lauchin’, and puir souls in the deid thraws warstlin’ the leelang nicht wi’ their bit ships — weel, it comes ower me like a glamour. I’m a deil, I ken’t. But I think naething o’ the puir sailor lads; I’m wi’ the sea, I’m just like ane o’ her ain Merry Men.’

1-What is the meaning of "There’s a sair spang o’ the auld sin o’ the warld in you sea; it’s an unchristian business at the best o’t; an’ whiles when it gets up, an’ the wind skreights "?

2-What is the meaning of "I’m thinkin’ — an’ thae Merry Men, the daft callants, blawin’ and lauchin’, and puir souls in the deid thraws warstlin’ the leelang nicht wi’ their bit ships — weel, it comes ower me like a glamour."?

3-What is the old sin? What is the relation between "the old isn" and "the sea"?

3) This is open to interpretation. Given the previous sentence ("Ye see, man, it’s defiance") and the following sentence ("I’m a deil, I ken’t") I would say that "the old sin of the world" is Satan's pride that led to his rebellion against God. The sea is also untameable and does not submit to rules.

1 & 2)
There’s a good measure of the old sin o’ the world in yon sea;
It’s an unchristian business at the best of it;
and at times when it gets rough, an’ the wind rages — (the wind and the sea are like brother and sister, it seems to me) — and the Merry Men, the daft rogues, blowing and laughing, and poor souls in their death struggles wrestling though the livelong night in their little ships — well, it comes over me like a spell.
I'm a devil, I know.
But I don't care about the poor sailor lads;
I'm on the sea's side,
I'm just like one of her own Merry Men.

Many Thanks for your explanation.