Since y'all weren't any help with 'dorck,' I tried to research it. Google was sure I meant 'dork,' but I persisted and wound up at definitions for 'thurrock,':

Noun
thurrock (plural thurrocks)

(nautical, obsolete) The hold of a ship; also, the bilge.

The Middle Dutch equivalent to this is 'dorck' exactly. So apparently the Scottish skipper wished he had thrown his mysterious passenger into the bilge water. And here I was thinking that 'dorck' was a variant of 'drink', and the skipper wished he had thrown him overboard.
=======
I didn't know marram was a kind of grass. It's a remarkable plant, growing on infertile sand with doesn't hold water.
============
Time for some more quotations.