How deep should a pond be to keep 3 huge Koi?

The Koi are each about 3 feet (90cm) long , 18 year old monsters, but the poor things swim in only 2 feet (60cm) of water, they could never swim vertically, the pond itself is very big, imagine 3 linked round sections - 24 foot (8M) dia each, but only 2 feet deep.

Is that alright? should it be deeper? do they need more depth for protection from the sun?

any info would be greatly appreciated

(i'll keep checking and answer any questions)

thanks :D