Sure, looking for any examples of how writing can be done underwater really, as there are so few that I can find
Not exactly an example from D&D or Pathfinder, but since the OP explained on a comment how they need any example of how to make underwater texts, I'll instead explain how I personally do things on my works where underwater writing is concerned.
Let the Rise of the Digital Mermaids begin.
Dealing with liquids while living inside another liquid is messy. Water, by the sheer nature of it, makes some fine movements a bit more complicated. It is harder to make things move underwater, it is harder to make them stop. It is harder to be precise, and writing with ink is a precise thing. A very precise thing.
That said, writing is all about storing information for later. As long as we can get information stored, it's all good. You don't need ink.
Anything that can be used to make symbols is enough. And that anything can be holes.
That's punched tape. It was used way back then to store information in a way computers could read later on. While not exactly as elegant of a writing format as Russian Cursive, it can get the job done when you can't reliably use ink. You just need some treated algae, water-resistant leather, or any other "sheet-like" material that you poke holes on, and you're set.
I personally use this method when writing stories or games that deal with one of my underwater races. It is easy to explain, it is easy to draw, and it is easy to make props out. It is alien enough to look like a different language, while being easy to translate if you have a dictionary at hand. More importantly, it is easy to demonstrate how a given race would use this method on their day-to-day lives: describing a scribe taking notes is not any harder than explaining a person using a pen-like device to poke holes in a sheet of shark leather bound to a wooden frame.
You can even do this live for your players, taking some paper and "poking down" some text in merfolkian live for then using a sharpened pencil.
More importantly, however, is that this method looks and feels very different from both regular western writing and eastern writing, while still being believable and practical. It enables one to create a very different, unique aesthetic, based entirely on patterns of dots, to races that are usually a little more than "elves with fins".