That line from Masefield has echoed in my head since I first read it as a teenager

It's the 'must' that nails it - it's a non negotiable call. If you don't, you die.

The rest of the line is also beautiful isn't it? From the middle stanza:

'I must go down to the seas again, for the call of the running tide
Is a wild call and a clear call that may not be denied'

I love the whole poem. He gets it.