Here's the way I sing it, nice easy chords:
Am G
The Diamond is a ship me lads,
Am G
For the Davis Straits she's bound
Am G
And the Quay it is all garnished
D G Am
With bonnie lassies round.
...
Chorus:
Am G Am
And it's cheer up, me lads
G Am
Let your hearts never fail,
G
For the bonnie ship The Diamond
D G Am
Goes a-fishing for the whale!