The Lake Scientist, also known as Dr. Mizumi in the manga, is a character from Ocarina of Time that resides in the Lakeside Laboratory and lives out his days studying marine biology. He studies the wonders of the depth of Lake Hylia's water, and researches the development of making medicine by mixing Lake Hylia's water with various unusual combinations.
While Link is in the midst of acquiring the Biggoron's Sword, at the Lakeside Laboratory, he finds that he needs the Eyeball Frog to create the World's Finest Eye Drops for Biggoron. After receiving it from King Zora XVI at Zora's Domain, he must bring it to the Lake Scientist on horseback for increased speed, as the Eyeball Frog spoils quickly.
Piece of Heart
Once Link wins the Golden Scale from the Fishing Pond, he can enter the Lakeside Laboratory and use it to dive down as far as he can. Link will touch the bottom and hear the Zelda chime. The Lake Scientist will be watching Link's diving secretly, and he will be so impressed that he rewards Link with a Piece of Heart.