Would it still take place in the sea?
I think it would be awesome if it takes place with Nemo and his dad who live in a wealthy city.
Nemo will get onto a boat to try and impress his friend's, but when his dad comes looking for him the boat starts heading out. Causing Nemo to become a stow away.
In a panic the dad tries to find a way to go after the vesil, but can't find any boats available for rent until tomorrow.
When the boat Nemo got onto reaches its destination. He gets caught and tossed off the boat by the angry captain. In which he ends up stuck in a fishing village. Where the locals mostly live amongst the sea. Many of whom teach him about sea life and how to survive like anyone else during his stay.
Back home, Marlin worries about his son Nemo. Which causes him to panic and decide that he can't wait a whole day to begin his search. Leading him to steal a boat and set out to sea on a mission to find his son.
Not to long after taking off Marlin realizes he has no idea how to sail a boat. That's when he discovers a young woman who was hiding aboard. He debates on taking her back to shore, but he doesn't want to lose the trail that might lead him to his son. Or risk getting caught by the authorites.
While debating with himself the woman reveals she knows how to sail the boat. And that she'll help him if he takes her with him. So against his better judgement he decides to bring her along. He also comes to find out her name is Dory and she has a terrible memory.
Thus starting their adventure. It's just a thought anyway...