No, I was just watching a doc on marine transportation and decided that Duolingo is the best place to comment on it. :) Of course I'm asking about the verb. If "מפליגה הרחק מכאן" has a second meaning, eg. that it is not the destination which is far away but the place of departure.
Well, in a nutshell רָחוֹק is an adjective, but הַרְחֵק an adverb (הֵם גָּרִים הַרְחֵק בְּר֫וּסְיָה they live far away in Russia). But the difference is not as clear cut, as רָחוֹק can be used adverbially too (הַתַּפּוּחַ לֹא נוֹפֵל רָחוֹק מֵהָעֵץ the apple does not fall far from the tree). The situation is similar with e.g.. טוֹב and הֵיטֵב. Both are examples of the adverbial use of an infinitive without -ל, formed from הֵיטִיב to do something well and הִרְחִיק to move something away respectively.