Time for a Swim
It’s the Final Day now, but you’ve just obtained a mighty sword and the Zora Mask. Time to get a little more use out of them. First, soar to Clock Town and visit the Trading Post in West Clock Town. Spend 30 Rupees to fill one of your Empty Bottles with Red Potion, then soar to the Great Bay Coast and put on the Zora Mask. Hold 
to swim north from the Marine Research Lab, heading for a lagoon.
Steer clear of the underwater Like Likes as you swim north and then east, into the lagoon. Swim down underwater, and examine the lagoon’s north wall to find four wooden panels, which Tatl flies over and inspects. Speak with Tatl, and she’ll hint that the wooden panels seem a bit out of place.
Back up and make some room, then hold 
+ 
and use magic power to swim toward the wooden sections at top speed. Ram into the second wooden section from the left to smash it apart, revealing a secret tunnel in the wall. Swim through the tunnel to begin your infiltration of a freaky pirate fortress.