Right. So this is the main acoustic part. It's just D, Am, C, G, but with the picking 
This repeats for the whole song.

Capo 3rd fret

         Humming    soft..ly     to    yourself

e----2-2-----2--------0-0-----0----0-0-----0----3-3------------|
B----3-3-----3--------1-1-----1----1-1-----1----0-0------------|
G----2-2-0h2-0--------2-2-----2----0-0-----0----0-0------------|
D--0-0-0-----0------2-2-2-0h2-2----2-2-0h2-0----0-0------------|
A--------------------------------3-3-3----------2-2---0--------|
E---------------------------------------------3-3-3-3----------|

     v ^     v        v ^     v    v ^     v    v ^

You watched the words change for someone else
Form an echo in a seashell
A drunken thought or a long farewell

And then tonight I'll dive into your eyes
Will we feel like this for the rest of our lives?
Just a bird, tied to a stone
Do I really want the stone to be thrown?

Under the sea, black and deep:
Everything I give is everything that I keep
So shake the leaves off this tree

And in the dark blue light
I watched the bottle sail into the night
Carried you home, shaking like a leaf
You couldn't swim even though you live by the sea

A flock of owls know your name
Will you promise me to stay the same?

Buried my hands on the old cold sand
I looked up and saw all the sunlight on you again

Night after night, I followed you home
Light after light, I followed your footsteps on the snow

(Under the sea, black and deep) x5

Under the sea, black and deep

Everything I give is everything that I keep x12