Unlock the Pelago Crocodile:
To find the Pelago Crocodile, simply look through your telescope on the morning before a full moon.
Unlock Apollo the Sunfish:
To locate Apollo the Sunfish, visit the Deep Hole at night. The sunfish should be above the Mouth of Truth.
Unlock Ancient Mother Whale:
You can find the Ancient Mother Whale in Adventures of the Deep by following the instructions below:
First, you need to complete the main story mode. You also need to have located every whale and dolphin in the game. Following these steps, purchase a totem pole and complete it's quest.
You will then see a mirage on the beach by the totem pole. Complete some of the special requests and maps until you unlock a carving of the Ancient Mother Whale. This indicates that the Ancient Mother Whale is now available off the North Coast of Canada.
Unlock Snorkel the Labrador:
There is a Labrador that you can bring home as a pet in Adventures of the Deep. Simply look through the telescope at Nineball Island - the game will tell you about the dog being washed up on the beach at Gatama Atoll.
Go to the Atoll (the northeast of the map) to find the Labrador.
Tip on Finding Rare Treasures:
Note: You will need to have the False Killer Whale with the X on it's forehead as a partner.
When you are diving with a partner, pull out your sea whistle. Blow it once to have your partner come to you. Wait for a few seconds, and if you have the False Killer Whale with you, it will answer and swim toward you. Blow the whistle twice without waiting, and the False Killer Whale will answer twice and swim away. Then wait. When you see the False Killer Whale point it's nose toward an area and stay there for about 5 seconds, swim in a circle, then repeat, get out your multisensor and point it toward the Whale. Your multisensor will go crazy and press A. You will find a rare object.
Try this out with other dolphins/whales. Doing this with the Orca helped me find the Moon Treasure, and doing this with the False Killer Whale helped me find Cleo's Treasure, Nesting Dolls, and other rare treasures.
Keep doing this, and soon your dolphin will become the ultimate treasure hunting partner!
Cortica River Secret:
Go to Cortica River Upstream. Go to the area with vines growing in the corner when you can see your body. Go as close as possible to the wall and the vines and to the surface. Press A and there will be a Gap in the wall. When you go back in the water you can swim through the wall. You're free! The area is infinitely deep and if you poke your head out of the water you can see from outside of the game and look in. Your diving partner (if you came with one) will stare at the spot where you escaped and make attempts to follow you. Pull up the map and the arrow that shows where you are will point at a land outside of the river. Have fun!
If it doesn't work on the first time, try, try again.
List of obtainable partners:
Here is a list of obtainable dolphins in the game and how to catch them.
Common Dolphin: Obtained when you go to Nineball island for the first time.
Arctic Porpoise: Obtained when you pulse the sick Porpoise in the Iceberg Caverns
Albino Bottlenose Dolphin: Obtained in Cireos Strait after meeting Finley
Flower Patterned Beluga: Obtained in NCOC* after playing "Simon Says" with her.
Red Horned Narwall: obtained in NCOC* after waiting for a red tusk to point out of the water
Pacific Whitesided Dolphin: obtained at Deep Hole after visiting it 4 times.
White River Dolphin: Obtained in Cortica River after following it with the Narwhall.
X-Marked False Killer Whale: Obtained in Valka Castle after chasing and cornering it.
Scarred Orca**: Obtained after buying a toy orca from nancy
Plump Rissos Dolphin**: Obtained after feeding it squid and an entire BANQUET of fish from nancy.
Commersons Dolphin with 8 stripes***: buy out all items from nancy and go to COTG and give it everything.
*: stands for north coast of canada
**: only obtainable after completing main storyline
***: only obtainable after reopening cavern of the gods