Finding 100 coins shouldn't be too much of a challenge in this level. Just remember not to let coins disappear and you'll have a good chance no matter what episode you choose. That said, choosing the right episode can make a lot of difference in the effort you'll need to put forth. In this level, don't forget to spray the round red enemies into walls rather than stomping on them to produce more coins, and try to flip on to the heads of the cactuslike pokeys for the same reason.

Blue Coins
The requirements list the earliest (or sometimes only) episode in which the coins appear, as well as any additional details.

Location: On a white wall facing town.
Required: Episode one.
Method: Spray the "M" to get the coin.

Location: On the side of the tower where the windmill spins.
Required: Episode one.
Method: Spray the "M" to get the coin.

Location: In town, adjacent to a building.
Required: Episode one.
Method: Drop off the roof or propel yourself up with a rocket blast or a Yoshi jump to grab the coin.

Location: The windmill mounted on a pole in the water just outside town.
Required: Episode one.
Method: There is one windmill that lies on the other side of the large white wall that closes off the town. Spray the blades with water to produce a blue coin.

Location: On back of the building closest to the hill, with two hollow towers.
Required: Episode one.
Method: Spray the "M" to gain the coin.

Location: In one of the towers on the building closest to the hill.
Required: Episode one.
Method: Jump and hover your way up and you'll be one blue coin richer.

Location: Underwater in the stream near the dam on the left side of town (from starting location).
Required: Episode one.
Method: Simply dive down and grab it.

Location: Visible underwater to the side of the piranha plant.
Required: Episode one.
Method: Dive down and come back up with another coin.

Location: On top of the white wall, near the waterwheel.
Required: Episode one.
Method: Spray the "M" to get the coin.

Location: In a small area created by the wall.
Required: Episode one.
Method: Spray the "X" on the opposite side of the wall to make the coin appear in the other section. Quickly make your way to the coin before it disappears.

Location: On the other side of the small walled-off area, facing away from town.
Required: Episode one.
Method: Hose off the "X" from the enclosed area and then make your way over to grab the coin. This is made much easier if you clean the "X" from the top of the wall.

Location: On the balcony of the highest building.
Required: Episode one.
Method: It's the second building to your left from the starting position. There's a pole with a platform on the roof. Use the hover function to drop off the roof and into the balcony. Then spray the left alcove with water.

Location: On a large stone pillar topped with grass behind the windmill.
Required: Episode two.
Method: Water the grass to uncover a shiny blue coin.

Location: On a large stone pillar topped with grass behind the windmill.
Required: Episode two.
Method: Water the grass to uncover a shiny blue coin.

Location: On the winding path leading up to the windmill.
Required: Episode three.
Method: A citizen of the island stands before you, covered in sludge. Wash him off with crisp, cool water and take a blue coin as payment for your service.

Location: Grassy ledge on the west end of town.
Required: Episode three.
Method: Make your way to the building farthest away from the start location. Climb the tower and point to the right of the dam. You'll see a blue coin on a grassy ridge. With the proper timing, a spin jump, hover, and dive will allow you to reach the coin. If not, simply bring the rocket pack to this location during episode four.

Location: On a brick outcropping on the eastern side of the level.
Required: Episode four.
Method: Follow the wall on the right from the starting location until you reach three brick steps. On the third is an enemy who will leave a blue coin after defeated.

Location: Under the bridge leading to the windmill.
Required: Episode four.
Method: The chain-link bridge to the windmill has been completed. On the underside of the bridge is a blue coin. Propel yourself up and move hand over hand to the coin.

Location: Under the bridge leading to the windmill.
Required: Episode four.
Method: The chain-link bridge to the windmill has been completed. On the underside of the bridge is a blue coin. Propel yourself up and move hand over hand to the coin.

Location: The first tower on the right side of the white wall.
Required: Episode five.
Method: Keep to the right of the starting point and hop on the first platform you come to. Move out on the wire and look to the right. You should see what seems to be a lookout post. Bounce off the wire and hover to your destination, where you'll find a dirty islander. Make with the high-pressure water and earn another blue coin.

Location: In the hills, on the group of platforms near the boss.
Required: Episode five.
Method: Use a rocket jump to hop to the top of the five platforms. Add the coin in the middle to your change purse and be on your way.

Location: Fluttering all about town.
Required: Episode six. You need to bring Yoshi along with you.
Method: Hop on Yoshi and keep your eyes peeled for a rare blue butterfly. Now eat it. Congrats, you're one blue coin richer!

Location: Next to a tall wooden pillar supporting a small windmill.
Required: Episode six.
Method: Use your method of choice to grab the blue coin suspended in the air.

Location: Underwater near the windmill.
Required: Episode six.
Method: There's a lone gold coin on the southern shore near the bridge leading to the windmill. By pressing Y, you should be able to see several more coins underwater. Take the plunge and follow the trail until you hit a blue coin.

Location: On the spot of grass where the boss was sleeping in the prior story.
Required: Episode six.
Method: Use your water cannon to inflate the clouds that have appeared over the gap leading to the boss's former resting place. Jump on the enlarged clouds and spray down the grassy area to reveal yet another spinning blue token.

Location: An "O" mark on the building in the back of town.
Required: Episode seven.
Method: There are two "O" marks in the town. Spray one and then rush to make it to the other before the blue coin disappears. Repeat.

Location: An "O" mark on the building in the front of town.
Required: Episode seven.
Method: There are two "O" marks in the town. Spray one and then rush to make it to the other before the blue coin disappears. Repeat.

Location: Atop the highest platform in town.
Required: Episode seven.
Method: The building in the front of town with a balcony has a lovely blue bird perched upon a platform. Stir him up, plant yourself on the platform, and then drown the poor creature with water until it expires and a blue coin plummets to the ground. Alternately, you could use Yoshi to devour the sapphire bird.

That's it! To find out how to acquire the two secret shines in this level, check the Secrets section.