The Ambergris Element

Seismic upheavals have left the planet Argo almost entirely covered by water. While exploring in an aquashuttle, Kirk and Spock are caught in an attack by a giant sea creature, and when they are found days later they have been transformed: they can now breathe only water. Realising someone must have changed them, they swim down and find the Aquans, an advanced undersea people descended from air-breathers. The Aquans fear surface dwellers, and the two officers are accused of spying and sent into exile. Rila, a sympathetic member of the ruling Tribune, helps them and points them to ancient ruins holding the secret of reversing the change. The cure needs venom from one of the sea monsters, and with help from younger Aquans they capture one. McCoy produces the antidote, the Enterprise's phasers help raise sunken land from the sea, and some Aquans consider a new life on the surface.