If you've never heard the song, definitely listen before reading this. Although I disagree with the filmmakers ending, he does a wonderful job of instilling a visual to better understand Mayer's tune.

Seriously...listen first.

The song narrates Walt Grace and his dream to build a "home-made, fan-blade, one man, submarine ride." apparent doubts from family and friends. It has an epic ballade like melody with beautifully implemented piano and snare drum. There's also a phrase used a number of times that can be interpreted different was through the songs playing. 

"When you're done with this world, you know the next is up to you."

My main qualm with the song comes with the interpretation of it's ending. It is incredibly ambiguous as to whether Walt Grace survives his final jaunt to Tokyo in his home-made submarine. A number of commentaries (including the film - not affiliated with Mayer) suggest he didn't complete his journey, based on the final few sentences of the song. 

Based on the language near the end, I believe Walt Grace completed his quest, but continued with a new life.

"One evening, when weeks had passed since his leaving
The call she planned on receiving finally made it home
She accepted the news she never expected
The operator connected the call from Tokyo
'Cause when you're done with this world
You know the next is up to you."

Walt's wife planned on receiving the call that he had drowned, because she boasted to her children that 'he was crazy'. Although she planned on receiving the call, she 'accepted the news she never expected' which probably meant the call came from Walt Grace himself. I'm assuming he told her about his plans to stay where he arrived. 

His friends at home now celebrate his journey. He is a home-town legend of a sort. I'm not sure if Walt's wife tells them he made it, but he continues a legend nevertheless.

I'd be interested in seeing rebuttals to my interpretation if anyone has any. I thought I had the meaning down in my head, then I woke up from a dream about Walt this morning at 5:30 like, "Walt made it.", which was much more comforting than an epic drowning at sea. I've been listening to it far too much lately.