Another option could be a mortal martial arts master who unknowingly stumbles upon Percy. The mortal is a master of a rare and ancient form of martial arts. At first, he is hesitant to teach Percy because he senses something different about him. But as he gets to know Percy better, he realizes Percy's good heart and decides to train him. This would add an interesting dynamic as Percy has to balance his demigod life with his training under a mortal.
Annabeth could also be a sort of mentor. She's very intelligent and could research about shapeshifting in the ancient texts. She could help Percy figure out how to control his ability better by coming up with strategies based on what she's learned from the books. For example, she might find a way to help Percy focus his mind to prevent unwanted shape changes.
One possible plot could be Percy discovers a hidden martial arts dojo in the demigod world. He decides to train there and quickly becomes a master. Then he uses his new skills to fight off monsters in a more unique way, combining his water powers with martial arts moves.
Nico di Angelo has a strong connection to the Underworld, so he's a prime candidate. He can already communicate with the dead and has some control over the shadows, which are related to death in a way. In fanfiction, he could be further developed to become the Master of Death, perhaps by making a deal with Hades or finding a hidden power within himself.
Madara Uchiha. He is extremely powerful with his Sharingan and Rinnegan abilities. Even if Naruto is a martial arts master, Madara's vast array of jutsu and his strategic mind would make him a formidable foe. He could use his space - time ninjutsu to avoid Naruto's attacks and then counter with powerful attacks of his own.
Another option could be Black Widow. She's very good at adapting to new situations and using her skills in different ways. For Percy, who is coming from a very different world, she could teach him how to be stealthy and use his powers in a more covert way. She could also help him with understanding the political side of the Avengers, like dealing with government agencies and such, which Percy may not be used to in his world of demigods.
Perhaps she's a sister who was thought to be dead but actually survived. She could have been corrupted by the experiences she had while being 'dead'. When she returns, she blames Percy for not saving her properly. This false blame leads her to betray Percy. She could start spreading rumors about Percy being a coward or a bad leader among the demigod community.
Eragon could be one of Percy's allies. Eragon has his own set of powers and knowledge of Alagaesia. Percy and Eragon could combine their skills, with Percy's control over water and Eragon's magic and sword - fighting abilities. They could also share their different cultural knowledge, Percy from the world of Greek mythology and Eragon from the world of Alagaesia.
A primordial being related to fire, like a forgotten Titan of fire, could emerge from the depths of Tartarus. It could be angry that the Olympians have been ruling for so long and sees Percy's new fire powers as a threat or a tool to use against them.
Percy himself could be a potential sword master in fanfictions. Given his skills with Riptide, he could further develop his swordsmanship under a new mentor. Another could be Annabeth. Her strategic mind would make her a great sword master as she could combine her intelligence with combat skills. Nico di Angelo is also a possibility. His dark powers could be incorporated into his sword - fighting style, making him a very unique and powerful sword master in fanfiction stories.
Well, in some Percy Jackson fanfiction, there might be other characters who are made to be the master of water. Maybe a new character is introduced who has some unique ability related to water that surpasses Percy's. However, usually, Percy is the main character associated with water powers because of his heritage. He can summon water, control its currents, and even communicate with water - based creatures. So, in most cases, when we think of the master of water in Percy Jackson fanfiction, Percy comes to mind first.