Learn Swedish: Äventyraren beger sig till havs. - The adventurer sets out to sea.

In old Swedish, the preposition till was always followed by the genitive case, which ends in -s. While this case system has mostly disappeared in modern Swedish, it survives in fixed expressions like till havs (to/at sea), till skogs (into the woods), and till bords (at the table).

Does till havs only mean movement ('to sea'), or can it also mean being 'at sea'?

It can mean both! Despite till usually indicating movement towards something, these fossilized expressions are often used for static location as well. For example, you can say both Vi beger oss till havs (We head to sea) and Vi är till havs (We are at sea).

What exactly does beger sig mean here, and could I just use går or åker?

Att bege sig is a reflexive verb meaning to set out, to depart, or to head somewhere. It sounds slightly more formal or literary than everyday verbs, which fits äventyraren perfectly. You could use åker (travels/goes), but går wouldn't work well here unless the adventurer is literally walking on the water!

Why is the definite form äventyraren instead of adding the full -en suffix?