Lake Scientist

The Marine Biologist is a recurring character in the Legend of Zelda series. His true name is never revealed. He is an eccentric old man who spends his time researching bodies of water and their many mysteries.

The Legend of Zelda: Ocarina of Time
The Marine Biologist performs his research in the Lakeside Laboratory, where he researches the many mysteries of Lake Hylia and the sea. He plays a role in the trading quest for Biggoron's Sword. Link must deliver the Eyeball Frog to him before it is spoiled. The old man initially assumes the frog is a present for him from King Zora XVI, as the eyes of Eyeball Frogs are known delicacies. He is disappointed to hear that it is needed to create the Eye Drops for Biggoron. Despite this, he extracts the proper ingredients from the frog and fills a vial with the efficacious eye drops. Link must deliver this to Biggoron before it spoils.

Also, in the Lakeside Laboratory, there is a deep aquarium. Link can dive into the water in this aquarium and after he resurfaces, the Marine Biologist, having watched him dive in secret, will judge his dive. Once Link retrieves the Golden Scale, he can dive to the very bottom of the pool. The Marine Biologist is so impressed that he gives him a Piece of Heart.

The Marine Biologist mentions, at one point, that Granny who owns the Kakariko Potion Shop in Kakariko Village, was a student of his.

The Legend of Zelda: Majora's Mask
The Marine Biologist bases his research from the Marine Research Lab located just off the Great Bay Coast. He has a good relationship with the Zora race, and promises to take care of Lulu's eggs for them. However, the eggs are stolen, and Link must retrieve all seven. Once all eggs have been gathered and released into the aquarium, the eggs will hatch, much to the astonishment of the Marine Biologist, and the Zora spawn will teach Link the "New Wave Bossa Nova".

In the manga
The Marine Biologist appears in the The Legend of Zelda: Ocarina of Time Manga, where he is given the name Dr. Mizumi. He is the one who tells Link that Rouro is a Watarara.