
The first line is often misquoted as "I must go down to the seas again." and this is the wording used in the song setting by John Ireland. I disagree with this last point. The poet himself was recorded reading this and he definitely says "seas". The first line should read, 'I must down ...' not, 'I must go down ...' The original version of 1902 reads 'I must down to the seas again'. In later versions, the author inserted the word 'go'.
Source: https://poemanalysis.com/sea-fever-john-masefield-poem-analysis/
Salt-Water Ballads (1902), "Sea-Fever"