Swim Until You Can't See Land

I think (as others do) that this song is about making efforts to forget the past, most likely a past relationship.

"We salute at the threshold of the North Sea in my mind And a nod to the boredom that drove me here to face the tide and swim."

I think this is the formal acknowledgement of the ending of the relationship and him realizing it's time to move on (or "swim").

"Dip the toe in the ocean. Oh how it hardens and it numbs. The rest of me is a version of man, built to collapse into crumbs. And if I hadn't come down to the coast to disappear I may have died in a land-slide of the rocks, the hopes and fears."

I think dipping a toe in the ocean is a metaphor for "testing the waters" again after the relationship, which can be difficult. I think a large part of him wanted to stay in the relationship and if he hadn't made up his mind to move on that he may have lived out his days in a relationship that didn't work.

Swimming until you can't see land I believe is simply representing taking the steps necessary to forget a significant other. The man v. bag of sand (as pointed out above) I think means do you have the guile to push forward and separate yourself from your ex or are you going to let them drag you down/back to them?

"Up to my knees now, do I wait? Do I dive? The sea has seen my like before though it's my first and perhaps last time. Let's call me a baptist, call this the drowning of the past. She's there on the shoreline throwing stones at my back."

He has made a few steps forward - should he keep going or turn back to the girl? He's been single before but perhaps never like this (after a difficult breakup). The baptism line to me is transparent - getting rid of the past and starting anew. The shoreline reference is probably that the girl is either close enough to turn back to (within a stones throw) or that she is taking active steps to prevent him from leaving (throwing stones at him).

"Now the water's taller than me and the land is a marker line All I am is a body adrift in water, salt and sky."

I think this is a success story. He is officially over the relationship (he is out of the shallows and the shore is distant/almost out of sight) and he is separated from his ex or anyone else ("all I am is a body adrift in water, salt and sky").