First bubble’s the easiest, It’s in the starting area, near Triche’s house. You have to trigger the intro scene first (by walking to the north to gaze at the surface), to be able to access it, but other than that, just walk down the steps to the north and you can grab it no problem.
Another very simple one to find, it’s in the northeast part of the beach area where you encounter Sinus and Joru.
This one’s on the left side of the kelp maze where you rescued Tae. The path isn’t super obvious, off of a dead end. It’s a very missable one.
It’s fairly missable since you can only enter the cave late in the game, and it’s never actually required in any of the routes to enter the Murky Water Cave through this route, but it’s not hard to find if you do go in there.
I believe on Moka’s route, you will be able to backtrack to all the bubbles at the end, as long as you don’t talk to her to begin the end scene, all the other routes will stop you from backtracking at some point.