Note: Guide credit goes to narcolepticshee of xbox360achievements.com. I take no credit for this, I'm simply reposting it for anyone that's having trouble looking for a certain shell.
To 'find' a Collectible Shell, you need to throw a gizmo at it so it hits it.
Coral Reef -
1. Look to the right as soon as you start, it is on the rock
2. You will pass under an arch - above the arch is number 2
3. After you have passed under the arch, you will turn right. Look to the right on top of two small round rocks
4. After number 3, you will see two columns of rocks to your right, look to the left here and around the corner is number 4
5. You will come to a T-Junction crevasse - look to the left, in the crevasse, for number 5
6. Immediately after number 5, turn round and look to the right of the big rock. It is above one of the green plants
7. After you have passed the rock where shell number 6 is, look to the left (behind the rock) for number 7
8. After you have passed the octopus on your left, behind the rock you pass over
9. Look in the direction you are moving and number 9 is in between the rocks ahead of you
10. After retrieving number 9, look to the left and it is under table shaped rock
Ship Graveyard -
1. As soon as you start, look to the right in between the green tall plants
2. After you pass in between the two large rocks (near the beginning) look to the left
3. As soon as you see the first shipwreck, look in the other direction to the big mountain - the shell is on the top
4. Look at the side of the next shipwreck - it is in between the bottom of the wreck and the top of the sandy bank
5. Next you will curve around the second shipwreck, When you reach the back of the ship, look next to the rudder.
6. Sometimes you will go over the ship and sometimes you will go in it, when you go over, look to the left of the top deck
7. Sometimes you will go over the ship and sometimes you will go in it, when you go in it, look behind the wardrobe to the left
8. When you exit the ship, look in between the wreck below you
9. There is a rock to your left after you exit/go over the ship and curve round. It is behind this rock
10. Further round, after you have passed round the back of the ship, look down
Deep Sea -
1. As soon as you start the level look down in between the rocks
2. Look to the right near the beginning, it is behind the statue
3. To the left of the entrance to the 'Atlantis' building
4. After the second left corner in the Atlantis building (just after the statue on your right), there are some arches to your left, look in between the arches and the fallen arch next to them
5. After the arch section, look in the direction you are moving and it is on the rocks, slightly to the right
6. You will go up an aztec pyramid style of steps - look to the left when you get on the platform
7. As you come off the aztec pyramid stepped platform, look to the left - number 6 is in between the legs of the giant statue
8. Sometimes you will go underneath the aztec building - when you do you will see three plinths in front of you, one with a pot, one with feet and one with nothing. Before you get to these pots, look to the left and it's just round the corner of the wall
9. As you are coming up to structure that looks like a well, look to the right in the distant rocks
10. Just before the Mermaid, look to the left behind the big block with the feet
Abyss -
1. As soon as you start, look at the pink tree to your left
2. There are three mushroom style plants to your right, number 2 is on top of them
3. Further down, there is another set of mushrooms to your right, number 3 is on top of them
4. Look down just after you see the red shrimp on the wall to your left
5. Look above you and right as you start rising, you'll see it in a plant above you
6. As you rise out of the Abyss you'll come across some flat ground, look down, you will see a purple spider-like plant - number 6 is on top of that
7. After obtaining 6, look immediately to the right and you will see it in between the large purple plants
8. To the left of the Kraken, on top of a red mushroom
9. After the Kraken, you will pass through two columns, one either side of you with a purple spider plant at the bottom. Look directly to the left after you have passed these
10. Just around the corner from 9, you will see a purple spider plant in the distance, number 10 is there
Volcano -
1. Near the beginning as you go over a rock, look down to the rock below - the first one is there
2. Look to the left as soon as you've collected number 1 and it is in the purple river
3. As you pass over the river, look to the right into the crevasse
4. After number 3, look in the direction you are moving in and you will see it above the hill in front of you
5. You will pass over another crevasse with a lobster in front of you. Look down and to the right for number 5!
6. After you've reached the top of the hill after collecting number 5 - look down to the left to see a crater full of plants - number 6 is in there!
7. Sometimes you will go left at the crevasse with the lobster and sometimes you will go right. When you go right, look round the corner to the right and down for number 7.
8. After number 6, look in the direction you are moving and you will see it in the distance in the ravine ahead.
9. You will come to a crevasse with a fork in it. Look at the bottom of the right fork.
10. Look up and left as soon as you have collected number 7