"...but like a fistful of brine, forever out of reach". Such a perfect analogy! Like fairy gold (https://en.oxforddictionaries.com/definition/fairy_gold).
And then "will-o-wisp" (which would seem to be an ill-fitting term for something as immense as a whale, but turns out somehow to work, maybe because of "shifting, submerging/A phantom on a silvery, sinking horizon"), another reference to the world of the little people... Such beautiful, otherworldly language! The last four lines gave me shivers :)
I see the will-o-wisp linked because the whale/ our desires are ultimately creatures of our imagination and no matter how large they would eventually dissolve or change to something else. If we indeed caught that whale, it would never be as we thought it. Really appreciate your thoughtful response, Bennett:) Thank you.
Yes! that comes across. The "fistful of brine" (still loving that image!) underscores it :)