There are a total of 26 Secret Seashells found in Link's Awakening. They are found all across the overworld, with a few being hidden in dungeons as well. After collecting twenty seashells you head to the Seashell Mansion to get the Seashell Sword.
Secret Seashell #03
Location: Mabe Village
Conditions: After obtaining the Deluxe Shovel.
You can get this seashell any time after purchasing the Deluxe Shovel for 200 rupees from the Town Tool Shop. After buying the Shovel, dig up the dirt in the bottom-right corner of the doghouse to reveal a buried seashell.
Secret Seashell #05
Location: Ukuku Prairie
Conditions: After obtaining the Deluxe Shovel.
Climb the stairs one screen right of the entrance to Key Cavern. Then head left one screen to find some patches of grass growing around a square spot of land. Dig up this spot to reveal a buried seashell.
Secret Seashell #07
Location: Ukuku Prairie
Conditions: After obtaining the Sword.
One screen right of the Seashell Mansion is an area with some bushes. Chop down the two on the top, and the second will reveal the hidden seashell.
Secret Seashell #08 (probably got this one)
Location: Richard's Villa
Conditions: After obtaining the five Golden Leaves and Roc's Feather.
After retrieving the five Golden Leaves from Kanalet Castle for Richard, he will allow you to take a set of stairs beneath his home to Pothole Field. After going down the stairs take the left path. Push the boulder into the pit and jump across to gain access to the chest where a seashell is hidden.
Secret Seashell #09
Location: Ukuku Prairie
Conditions: After obtaining the Sword, the Deluxe Shovel and the Power Bracelet.
Travel south towards the Cemetery to Ukuku Prairie and you will notice a rock blocking a staircase. Move the rock with the Power Bracelet and then head down one screen. Cut down the bush there and dig in its place to recover a seashell.
Secret Seashell #10
Location: Ukuku Prairie
Conditions: After obtaining the Pegasus Boots.
Right after leaving Mabe Village to the east (where you cut through the bushes and move the rock), head down one screen. There is a Telephone Booth and a tree here. Ram the tree with the Pegasus Boots equipped to reveal a hidden seashell.
Secret Seashell #18
Location: Martha's Bay
Conditions: After obtaining the Sword and the Flippers.
Go near the House by the Bay and then head west where you will see an island with a bush. Swim over to the island and chop down the bush to retrieve the seashell.
Secret Seashell #20
Location: Martha's Bay
Conditions: After obtaining the Deluxe Shovel.
Travel two screens south of the entrance to Animal Village and one screen left afterwards. Then, go up one screen across the bridge where there is an owl statue. Dig the ground to the left of the statue to unearth a seashell.
Secret Seashell #21 (unsure if got this one)
Location: Southern Face Shrine
Conditions: None.
Go to the Southern Face Shrine north of Animal Village. In the northern center part there is a staircase below the Armos in the lower-right corner. Make it move away from the staircase below by just touching it or killing it as well. Go down the stairs and open the chest for a seashell.
Secret Seashell #23
Location: Kanalet Castle
Conditions: After obtaining the Power Bracelet, the Ocarina, and the Legendary Rooster.
Late in the game you will have to revive a Legendary Rooster under the statue in Mabe Village. Once you do, head over to Kanalet Castle. Fly west from the gate across the pits. Go down the stairs and open the chest for a seashell.
Secret Seashell #26
Location: Eagle's Tower
Conditions: None.
From the start of Eagle's Tower go left one screen and then up one. If the blocks are in the way of the staircase, locate a switch to change them out. Go up the staircase and continue down one screen. Jump in the top-left pit in this room and you will land on the walkway next to the wall on the floor below. Go up two screens onto the walkway and you will come to a chest. Open it to receive the seashell.