Ron and Hermione are the most likely ones. They are his best friends and have always been by his side through thick and thin. They would probably follow him without hesitation and use their own skills, like Hermione's vast knowledge and Ron's loyalty and practical skills, to assist in the ship - stealing adventure.
It's possible that Luna Lovegood could be involved. She has a unique perspective on magic and might offer some unorthodox but effective solutions during the ship - stealing escapade. For example, she could have some knowledge about the magical properties of the sea that could help them navigate more easily. Hagrid might also play a role. He has a great deal of knowledge about magical creatures, which could be useful if they encounter any sea - based magical beasts while on the ship.
Maybe Harry Potter steals a ship to escape from some dark wizards who are chasing him. He uses his magic skills to sail the ship stealthily and heads towards a hidden island where he hopes to find a powerful magical artifact to defeat the dark forces.
There could be some new characters introduced in the fanfic. Maybe a powerful wizard from a far - off land who has heard of Harry's plight and decides to help. Or perhaps a group of outcasts in the wizarding world who see Harry as a symbol of their own struggle against the unjust system and come to his aid.
Some of the Hogwarts professors could also assist. Professor Flitwick could use his charms expertise to add special touches to the city - ship, like making the doors open with a unique charm. Professor McGonagall could use her transfiguration skills to transform materials into the needed shapes for the construction. And Neville could also play a role. He has grown into a powerful wizard, and his knowledge of herbology could be used to create a beautiful and functional garden on the city - ship.
He might meet a mysterious old wizard who has been traveling the seas for centuries, gathering knowledge of different magics. This wizard could be a great source of wisdom for Harry.
Hermione would surely be one of the first to help. She is highly knowledgeable in magic and would likely try to use a healing spell or find a potion to stop the bleeding.
Another possible antagonist could be a dark wizard who has a special interest in the wands that Harry is stealing. This dark wizard might want these wands for his own evil plans and sees Harry as a competitor. So, he would try to stop Harry from stealing the wands while also trying to get his hands on them himself.
Dumbledore, if he were still alive in the fanfiction, would likely have some wise words and advice for Harry. He might have some secret knowledge about dragon transformations that could be crucial in helping Harry. Also, Neville could come in handy as he has proven to be brave and resourceful, and he might be able to find some rare plants that could be of use to Harry in his new form.
There could be lost wizards who got stranded on the ship for some reason. Maybe they were on an expedition and something went wrong.
He could have adventures like exploring uncharted magical islands. Maybe he'd encounter new magical creatures and have to use his wizarding skills to either befriend or fend them off. For example, he might come across a group of mermaids who guard a powerful magical artifact on an island.
Most likely Hermione. She is always looking out for Harry's well - being and would want to make sure he gets the flu shot properly. She would also be curious about the Muggle medical procedure.