Translation:He was swimming in the sea all of last year.
You would never say this, it is weird and slightly illogical.
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.
At least the English sentence doesn’t suggest that the swimmer didn’t get out of the water for the whole year, just that it was a regular activity for the whole year. Likewise for the Danish sentence?