Rather freely formulated.

It would be helpful to have a little more context. The preceding sentence, in particular, would clarify if they are talking about the speed of the boat or something else. My suggested translation of "merialue" is very loose, based on the guess that they are encouraging the reader to speed up in areas where there's more room to zoom around. Depending on the context, though, it may be necessary to use a more precisely corresponding navigational or jurisdictional definition. Just as a start if you need to go in that direction, "merialue" doesn't turn up a direct English translation in Eurodicautom, but is defined as "alue, johon kuuluu useita satamia" (an area that includes many harbors).