Pinnacle Rock

From Zelda Wiki.org

Pinnacle Rock is a place found in Majora's Mask in the Great Bay, and is full of mist and signs found on the sea bottom. Beyond the signs, the lair of the Sea Snakes can be found, a deep hole in the sea filled with predators, all of them Serpents. There are eight serpents, three will have the Zora Eggs you are looking for, two will have chests containing 20 Rupees ,one will have the lost Seahorse, and the two remaining will have nothing.

The way to get through Pinnacle Rock without the seahorse's assistance are:

Stop at the first sign, go to the second one on the right, go one forward, head to the left until another sign comes into sight, one up, one right and then into the lair.

Getting out this same way is quite difficult; it is far easier for Link to simply swim out a random way and magically wind up back at the entrance to Pinnacle Rock.

It is encouraged that you collect the seahorse not just because it's easier to get in, but because you get a heart piece by releasing the Fisherman's seahorse by the lost seahorse.