"... and can be found on board a ship, ..."

On what? Cleary, the improper punctuation is is key or code. As Cluin has clued into, the writer has instructed boarding a ship, if certain conditions are met, including, "... is slimey in the extreeme, ...".

Note the inclusion of a second "e" in "extreeme". Perhaps this is a signal that other letters have been added or misaligned. As McGrath has pointed out, "Anyway, rat's aren't slimy as a rule." Ergo, might the author have been conveying that the subject is rather English?

We need Tom Hanks, quickly.