First of all, you need a water gun.
If you don't have a water gun, explore the land at night and maybe you'll meet someone who is willing to give you one.




And that's it. Now you're ready for your hunt for this little secret!

There are 5 shells lying on the ground across the land near water. I think they spawn randomly so the best way to find them is just walking along shores (or rather jumping as it helps to spot them earlier)
As soon as you get near, they will close, but don't worry! Splash them with water so they will open and you'll be able to take the pearls. (UPD: I think they aren't spawning open now, I found them all closed on my second playthrough)
These pearls will just disappear as you click on them (there should be a circular pointer when you hover over a pearl) without making any notes in your textbook (or at least I didn't find them anywhere) but there will be a pencil icon on the bottom left corner as when you catch a bug.
When you find all of them, there will be no more shells.



This is how the cursor looks like showing that you can collect the pearl


As soon as you collected the last one, there will be a tiny animation making your water gun golden!

There are some piles of dirt in the flower garden.


These are actually statues covered in mud, so you have to splash them with your handy water gun too and they will reveal themselves, all shiny and clean.



Although they're all very nice and spectacular, we're looking for a particular one looking like Venus (the woman without hands)


The necklace is showing how many pearls you've got. I have a feeling that the colours may be a hint to show you where to look but I'm quite bad at navigation in this game so it's just a guess!

Here I'm missing 2 pearls


As soon as you finish collecting the pearls they all will be coloured and there will be an eye icon showing that you can now interact with the necklace.


Now you just need to click on your finished necklace and a friend will give you a pair of boots.
And you're all done, good job! Enjoy your lightning speed (: