This could be a unique premise if they made more connection with the lake... Like, why becoming a lake? It's so random to have just an ordinary student dozing off in the classroom to suddenly become a lake.
Why not make the MC someone who has a connection with water to begin with? A person who loves to spend time at a lake? Or someone that had a family member drowned in the lake, so when he became one, he had a reason to be helpful so other people wouldn't meet the same tragedy?
This is something the author came up with just for the sake of wanting to be a different isekai but not commit to actually making it interesting...