Seabound spellcasting

A lone orca happily swam through the ocean, humming to himself as he looked for his next meal. Some tuna would certainly be nice, they were his favorite... But, eventually, he encountered something quite a bit more interesting: Ruins!
Excited at the prospect of exploring them, the orca surfaced for a breath before diving down, swimming among the columns and assorted debris. Some had markings on them that seemed fairly consistent - writing. Though, of course, he couldn't read. That didn't stop him from theory crafting as he looked around; a place like this certainly couldn't have been made by cetaceans, lacking any digits on their flippers, so who did? They must have been aquatic, whoever they were.
After a few minutes, another break to breathe, and accidentally scraping himself on some rubble, something caught the orca's eye. On a fairly tall pedestal, there was a thick rectangular object of some kind, the red, blue, and gold colors of it standing out against the grey of all the stone architecture. Upon getting a closer look at it, it had a water droplet design on top, and it seemed like it could open. The orca had heard of things like this; they were called "books", and they originated from the land. But, he also knew these didn't normally withstand water this well...
A bit awkwardly, the orca pulled on the book's cover, opening it. It was filled with strange symbols, and diagrams depicting equally strange bipedal beings, and as always, he didn't understand any of it. But, after a moment, the book's contents almost seemed to shift, somehow now communicating concepts he understood. Even the diagrams now depicted orcas!
The book described specific combinations of gestures and thoughts - "spells", it called them - that could somehow have strange effects on the world... Very much curious, the orca read through the first set of those, described as being able to control water, and decided to give it a shot. Holding his flippers out to the side, he closed his eyes and focused in just the way the book described... Then, he felt strange, as if something he didn't know he had got slightly drained. Maybe the spell had worked?
He opened his eyes again, and was greeted with a strange sight; his flippers both had several bands of blue energy around them, moving with them. The orca tried to touch one, but the band bent out of the way... And as he moved his flipper, a sudden current blew the book off the pedestal! He quickly grabbed it again as he figured out what was happening; as the book said, he was controlling the water around him!
Holding the book so it wouldn't get blown away again, the orca kept experimenting. With waves of his flippers, he could cause currents where and how he wanted, knocking some bits of rubble around. A particular forceful wave even pushed a broken column some distance! But, each time he made a current, the orca felt that draining sensation, especially with the stronger one.
He decided to look into what that was before he'd continue. Setting the book down on the pedestal again, he used more careful waves to turn its pages, this being substantially easier than directly using his flippers. After a bit of searching, he found a more basic explanation he'd missed.
The spells in this book used something called "mana", a form of energy found in all living things and the environment itself. Any he used would be recovered from the environment over time, but he could only have so much in him at once. The book also warned not to keep using spells without enough mana, but didn't explain why...
Knowing he'd probably find out at some point, the orca started flipping through the book again, looking for another spell that interested him. He could already do an alright job at fighting, so quite a few of the book's combat-based spells weren't all that important. One that could apparently turn the user's body to liquid for a while certainly caught his attention, but he could already tell that was WAY beyond him.
Eventually, he settled on a simple healing spell. While he wasn't quite certain how it qualified as water magic, it would definitely be helpful to learn. After a couple minutes of studying, and another break for air, he felt confident he could do it, and he had a good way to test it out. Angling himself to look at where he'd gotten scraped earlier, he focused on the healing spell. The blue bands around his flippers gained a green tint, and he held one over the injury. The orca felt his mana draining more, and sure enough, his scrape faded away!
While he was just about satisfied for now, he wanted to try out one more, something more advanced this time. After some more looking through the book, he found the one; something that could let him breathe underwater! Making a current bring the book with him, the orca went up to being just below the surface, just in case he did this wrong. This one took a while longer for him to get a feel for, and he felt his mana very slowly depleting over time.
With about half of it left, he felt confident he could get the spell to work. It was quite a bit more involved than the others, but after the gestures and thoughts, his chest gained a blue glow as he felt his mana drop by a substantial amount. Now, all that was left was to convince himself to breathe... It felt deeply wrong to him, the thought of taking a breath while fully underwater, so it took a while, but eventually he was ready. Still nervous, and feeling a need for air, the orca finally opened his blowhole and pulled in water...
...And, nothing happened. It simply felt like air! Except, even more of his mana was used up, leaving him without much left. After confirming he did indeed have his blowhole submerged, the orca smiled, happy to be able to breathe without surfacing! Oddly, though, the need for another breath was making itself known far more quickly than normal, and in just 20 seconds, he had to inhale again, drawing in more of the newly breathable water.
This breath took the last of his mana, and a dull ache spread through his body. This must have been the consequence of not having enough mana... But, it wasn't so bad, he could keep going. Swimming just under the surface, the orca soon took a third breath, and whined as pain shot through his body, the ache becoming much worse. Now he could tell, if he kept this up, he'd end up seriously hurting himself...
With that realization, he quickly surfaced, exhaling all the water before ending the spell. The ache was still there, but breathing normally didn't make it worse. He definitely needed a break to recover... With the spellbook held in a flipper, the orca swam off, looking forward to the changes his new magic abilities would bring to his life.