Master's Plaque
The Master's Plaque is a quest item from Oracle of Seasons. It belongs to the Master Diver, and Link must obtain it to prove himself worthy of becoming his pupil. Only then does he obtain the Flippers.
The plaque is located inside a cave above a waterfall in the Sunken City, and Link can only reach it with the help of Dimitri. After Link hits the four eye statues simultaneously, a staircase leading to the plaque will appear. Once Link acquires the plaque, he must return it to the Master Diver in exchange for the Flippers.
References
" I am the master diver! To become my pupil, go to the next cave and open all four eyes in a single stroke! And bring back proof that you have done so!"
" Ohh!!! That's my Master's Plaque! You've passed my test! Very well, then! I accept you as my pupil! Take these Flippers!"