Learn Swedish: Han är pirat och beger sig till havs. - He is a pirate and sets out to sea.

This is a fossilized genitive form, which is exactly what this lesson covers! Historically, the preposition till forced the noun after it to take an -s ending. While modern Swedish doesn't do this anymore, the old rule survives in fixed environmental expressions like till havs (to or at sea), till skogs (into the woods), and till bords (to the table).

Is there a difference between going till havs and going till sjöss?

Both are fossilized expressions meaning 'to sea', but they have slightly different flavors. Att gå till sjöss is a strong idiom for becoming a sailor or joining the merchant navy. Till havs refers more literally to the physical environment of the open ocean, which makes perfect sense for a pirate setting sail.

Why isn't there an en before pirat?

When you state someone's profession, role, nationality, or defining affiliation using the verb att vara (to be), Swedish usually drops the indefinite article. Even though being a pirate is an unconventional job, it is treated as his profession here, so it is just Han är pirat instead of Han är en pirat.

How does the verb beger sig work in this sentence?