Translation:He was swimming in the sea all of last year.
The sentence would translate to 'he walked and swam in the sea (ocean) all of last year'. A better version would be 'Han svømmede i havet hele sidste år' In general, many of the sentences in this lesson contains 'stod' (stood) or 'sad' (sat), which are often plain wrong or unnecessary, and would be omitted.
I am a native dane, and can say for certain that the sentence is incorrect. As mentioned above, the translation from danish completely omits 'han gik' (he walked) in the english version. Using the words above, the sentence could be restructured to: 'Han gik I havet og svømmede hele sidste år', which would translate to 'he went into the sea and swam all of last year'. 'Gå/gik' in danish is used just like 'Go/went' in english (Jeg gik i seng/I went to bed).