Fushigi no Umi no Nadia

Synopsis

As the Nautilus resumes its pursuit of Gargoyle, both Marie and Nadia become seriously ill from a tropical fever that will claim their lives in two days time if they are not cured. The only hope for a cure is in the depths of Reef 64. Alarmed to learn that Nadia is in danger, Nemo decides to change course, much to Electra's infuriation. Accompanied by Nemo, Jean, Sanson, Hanson, and King set off into a dangerous trek through the underwater domain of Reef 64. They find the herb in a cave situation on the opposite side of a cliff face. On their way back, however, they are attacked by a monstrous eel called a "Dinicthys." Jean and King bravely attempt to draw the Dinicthys away from their position by throwing their diving suits' lights into a void. Thanks to help from Nemo, they succeed, and the girls are cured.

(Source: Wikipedia)