The trick to this song is staying on top of the triplets when you're strumming. 
Some of the chords are ?transitions? so listen for the changes.  On the C-Cadd9 
change, you just drop your pinky down right before the Em. 

Enjoy!

G            C  Cadd9  Em       D        
I believe submarines
G            C  Cadd9  Em       D  
Underneath deep blue seas
G            C  Cadd9  Em       D  
Saw the flags: Japanese
G             C  Cadd9  Em       D  
No one will believe me

Em                     D 
Ran back to the town bar and
G
I told the people how
Em                      D
I had seen the submarine and
C
Everyone laughed aloud 

G    C  Cadd9  Em       D  
G    C  Cadd9  Em       D  

G    C  Cadd9  Em       D  
I did spy periscopes
G            C  Cadd9  Em       D
Dead of night on the eastern coast
G            C  Cadd9  Em       D
The police made them jokes
G            C  Cadd9  Em       D
Told me I'd seen a ghost

Everyone thinks I'm a liar
No one knows the truth
If it was a bigger fire
I would be on the roof

Ships will rust in Baldwin Bay
(Let me go!)
No one trusts what I say
(I don't know!)
Oh my God, no one paid
(Attention!)
Overnight my hair turned gray
(Oh oh oh!)

In the end it boils down to credibility
I had none, so I will die with the secrets of the sea

(Submarine! Can't find me! Submarine!)

Submarine...

(Submarine!)