Dropping off the Eggs
With all four Zora Eggs successfully recovered from the Gerudos, you’ve completed your infiltration of the Pirates’ Fortress. Soar back to the Great Bay Coast, and then climb the ladder and enter the Marine Research Lab.
The old scientist in the lab informs you that the Zora Eggs can only hatch inside the big aquarium here. Climb up the nearby wide metal ladder, move to the center of the big aquarium’s barred top, and then dump out all four of the Zora Eggs that you recovered from the pirates into the tank.
The Fisherman’s Catch
You’ve found four of the lost Zora Eggs, but there are still three more to collect. These last three are somewhere deep underwater, and you’ll need a little help to find them. Exit the Marine Research Lab and swim to the east beach, then enter the Fisherman’s Hut, which is located next to the Oceanside Spider House.
Inside the Fisherman’s Hut, 
-Target the shiny gold Seahorse in the fishbowl near the Fisherman. The little Seahorse begs that you take him back to the waters near Pinnacle Rock, and the Fisherman is keen to notice your interest in his little friend.
The Fisherman will give you the Seahorse, provided that you give him a pictograph of the “female pirates” in exchange. As luck would have it, you just happen to have one. Show the Fisherman the pictograph that you took of a Gerudo guard inside the Pirates’ Fortress, and he’ll happily hand you the Seahorse in trade.
Seahorse
You got a Seahorse! Hurry—take it to its home at Pinnacle Rock.
To Pinnacle Rock!
Pinnacle Rock isn’t far. Put on the Zora Mask, and swim to the two tall rock pillars that stick out from the bay to north of the Marine Research Lab.
As you near the tall rocks, a Zora emerges from the water. He hints that a gold-colored fish might be able to help you navigate the murky waters of Pinnacle Rock. Sounds like that Seahorse is about to come in handy.