You can get the majority of the Blue Coins in Pinna Park during Episode 8, but there are a few that are specific to different episodes. To get things out of the way, Episode 2 and Episode 4 will prohibit you from entering the actual park, so any Blue Coins found inside cannot be gotten during those episodes.
Blue Coins #1-#6
To start with the Blue Coins on this level, you will be going to Episode 2. In this level, there is a cannon shooting Bullet Bills at you from the other side of the beach. On this beach, there are overturned baskets that are indestructible unless hit by the Bullet Bills. So, your goal will be to lure the purple Bullets to the baskets to make them explode, getting the Blue Coins underneath. You will have to lead some of the Bullets to the baskets, as they are in odd spots.
Blue Coin 7
Pick Episode 6 for this coin and go inside the park. Find the tree that looks like a Pianta and behind it will be the Yoshi egg. Find the fruit it wants and return it to hatch the egg. With Yoshi in your possession, head towards the giant pool of water and look for the beach just over the wall. Near the palm tree with the flowers next to it, there will be a blue butterfly, so eat it to spawn the Blue Coin.
Blue Coin #8 and #9
There are four coins found on the beach at the beginning of the level. These two are hidden marks on the beach, which means you will want to spray everywhere you can until you get the hit markers. One is by the tree with the Yoshi egg, while the other is a little closer to the cannon.
Blue Coin #10 and #11
The other two coins on the beach will require Yoshi, with the egg being under the tree in the middle of the beach. The fruit is hidden underneath the sand and the only way to get it is by spraying the beach with water. Once you’ve gotten the necessary fruit, hatch the egg and bring Yoshi over to the sunflowers. There will be two blue butterflies here, so eat them both for the Blue Coins.
Blue Coin #12
Inside the park, there will be a ride right there with clam shells. Look for the green one and spray it to open the shell, where the Blue Coin will be hiding.
Blue Coin #13
Near the shell ride, there are some stairs and underneath it will be a Blue Coin in a semi-caged area. Get in the water, with your Hover Nozzle, then jump out underneath the coin and hover until you get it.
Blue Coin #14
Next to the stairs where you found the previous coin, there is an orange-colored archway. There will be a piece of graffiti on it, so clean it to get the coin.
Blue Coin #15 and #16
Pass through the orange arch and up the slope a little bit will be a tree shaped like a Pianta. There is a X-mark graffiti on it, which means there’s another one somewhere within the stage. It will be found on the other side of the white wall, through the opening, by the large pool. When starting at the tree, get as far away as possible before spraying the X. For the other one, you can get on the higher green ledge.