Kenneth Bulmer 
City Under the Sea 

Jeremy Dodge knew the Earth would face starvation if it were not for the new science of "aquaculture". With the world’s population numbering many billions, only the extra food being cultivated on the bottom of the sea could feed everyone.But, like the rest of the surface-dwellers, Jeremy did not know what a vicious monopoly underwater cultivation had become. That is, until the dreadful moment when he himself was kidnapped and dragged beneath the depths.And there he was to learn that just making his own escape would not be enough — he would have to save mankind from the tyranny of a new race of water-breathing human monsters!