This isn't a nautical term or anything, but here in Michigan where we have a lot of lakes and boats, most people would say just 'goes on' or 'goes out on,' if this is what the boat normally does. (So I think the translation is perfectly fine. On a river, you'd say 'goes down,' 'goes up,' 'goes up and down' or possibly 'goes on' the river.)
e.g. 'The freighter goes on Lake Superior, through the locks and then on Lake Huron.' 'They go out on their boat on Lake St. Clair on the weekends in the summer.' 'You can see that yacht going out of the channel onto the lake now.'
I couldn't find anything better than travels (apparently sails would work for many vessels without sails, but not for a little motor boat). I also like is on the Baltic See then both
åker could be accepted, like in those sentences with towns and countries being somewhere, where both
ligger are accepted.
As a native English English speaker (and having read much of this discussion 10th Jul 18), I'd offer the following. 1. Given that the Swedish uses a verb of motion, I would translate using the verb 'to sail' if the boat/ship had sails or was of significant size and powered by a motor/engine. 2. The other alternative suggestions in this discussion use verbs of location or existence, which are fine, if that's what you want to convey. 3. The verbs to go or to travel would work in particular situations but each sounds clumsy or infantile if used as the norm.
Please could “the Baltic” be added generally as a translation for “Östersjön”, not just “the Baltic Sea”? A few exercises accept it as an alternative, but it seems by default Duo doesn’t have it as a translation of the name. In English it is very common to just call it “the Baltic”; indeed, a quick google ngrams search suggests that until recently it was significantly more common than “the Baltic Sea”.
(I’ve reported this separately on several exercises, but just commenting here to explain the reason a bit more.)
The typical way to render this in British English is The boat sails the Baltic Sea (and, incidentally, whether the boat has an actual sail is irrelevant), but I can see how that's problematic for translating in the opposite direction, replacing the more general verb and removing the preposition.
The preposition can be left intact without sounding too odd, but sails sounds a lot more natural than goes. Given the problem of mutual translation, however, perhaps the odd-sounding translation is best left alone.