Learn Swedish: Kanske havsströmmen kommer från en annan kontinent. - Maybe the ocean current comes from another continent.

You are right to notice that! Normally, starting a sentence with an adverb forces the verb into the second position. However, kanske is a famous exception in Swedish. It often acts more like a subjunction, allowing the subject (havsströmmen) to come before the verb (kommer). While Kanske kommer havsströmmen is also grammatically correct, breaking the V2 rule with kanske is extremely common and natural.

How would I use the relative adverb 'varifrån' to talk about where this ocean current comes from?

You could form an indirect question like: Forskarna undrar varifrån havsströmmen kommer (The scientists wonder where the ocean current comes from). Here, varifrån replaces the need for a separate från at the end of the clause, neatly bundling 'where' and 'from' into one connecting word.

What is the purpose of the extra 's' in the middle of 'havsströmmen'?

It is a linking 's' (foges) used to glue the two nouns hav (sea/ocean) and strömmen (the current) together into a single compound word. This is very common in Swedish compound nouns and historically stems from the possessive 's', making it literally 'the sea's current'.

If I wanted to say 'another country' instead of 'another continent', how would the word 'annan' change?