[Verse 1]
Ooh, my Adeline, ooh
Down in Tasmania
Where the devil's jaws are far too weak
To tear you away
[Chorus 1]
Ooh, I wish you well
I wish you well
I wish you well
I wish you well
[Verse 2]
Ooh, my Adeline
I watch your morning swim again
Ooh, Kodachrome blue sky
Strokes from patch to warm patch
My sweet Adeline
[Chorus 2]
Ooh, I wish you well (My Adeline)
I wish you well
I wish you well (My Adeline)
I wish you well
[Verse 3]
And the old triangle went jingle-jangle
All along the Royal Canal
My Adeline was swimming; sweet Adeline was singing
To the tune of the Royal Canal
[Bridge]
She's a swim, swimmer
Swimmer, swimmer
Swim, swimmer
Swimmer, swimmer
[Verse 3]
And the old triangle went jingle-jangle
All along the Royal Canal
My Adeline was swimming; sweet Adeline was singing
To the tune of the Royal Canal
[Chorus 3]
I wish her well (My Adeline)
I wish her well
I wish her well (My Adeline)
I wish her well
I wish her well
Ooh, my Adeline, ooh
Down in Tasmania
Where the devil's jaws are far too weak
To tear you away
Ooh, I wish you well
I wish you well
I wish you well
I wish you well
Ooh, my Adeline
I watch your morning swim again
Ooh, Kodachrome blue sky
Strokes from patch to warm patch
My sweet Adeline
Ooh, I wish you well (My Adeline)
I wish you well
I wish you well (My Adeline)
I wish you well
And the old triangle went jingle-jangle
All along the Royal Canal
My Adeline was swimming; sweet Adeline was singing
To the tune of the Royal Canal
She's a swim, swimmer
Swimmer, swimmer
Swim, swimmer
Swimmer, swimmer
And the old triangle went jingle-jangle
All along the Royal Canal
My Adeline was swimming; sweet Adeline was singing
To the tune of the Royal Canal
I wish her well (My Adeline)
I wish her well
I wish her well (My Adeline)
I wish her well
I wish her well

I just think his girl is somewhere in Australia, running from something, keeping herself safe. probably missing her. There is a reference to an old song The Auld Triangle. https://www.youtube.com/watch?v=PWilmT36RGc

Sublime song, especially the second half where all the elements come together . Alt-J at their best here.
It seems to me to be a memory, all in the past tense. Perhaps reminiscing about a girl he met in Tasmania, he remembers watching her swim. Perhaps he regrets not spending more time with her, saying something to her... but it is too late now so all he can say is "I wish you well"
If you haven't seen the video it is worth a watch...
Love to all you fellow Alt-J fans ;-)