"De syngende mænd var på et skib langt ude på havet."
Translation:The singing men were on a ship far out on the sea.
Both yes and no. According to Gyldendals dictionary the english word 'ocean' can also translate to 'hav', as in the danish translation for The Atlantic Ocean; "Atlanterhavet", The Pacific Ocean; "Stillehavet" and so on. So 'ocean' should be perfectly acceptable. (On a side note for those interested, the word 'sea' is also at rare times translates to 'sø', mostly in fixed expressions like 'søens folk'; sailors, or 'Østersøen'; the Baltic.)
I think it may accept, but if you hover your mouse over it you can see the answer.
Sø in general means lake; the different meaning in Østersø is due to a switch between the meaning of lake and ocean in older germanic languages. It is just a linguistic relic here, more relevant in Dutch or Lower German.