@Mira995: I still want to think that it's actually a strong spirit by the world's standard, just that Jeanie is such an Unreliable Narrator that she fail to notice things + most of the time the people who know aren't around to point it out for her (and the teacher was so dumbstruck he didn't say anything)
Like, the amount of water created initially was rather massive, so maybe Undine spent so much of the spell's energy condensing it down to 'just' water ball level that it didn't have much to spare in giving it velocity.
Or maybe what is shown is indeed a water ball, but Undine only used that much power because Jeanie asked for a water ball. (since spirit's logic might not be the same as the master, see Ray and the 1%)
Even when Undine came out, it was Jeanie who called it Undine, so maybe she named it Undine, and not that the spirit is of Undine type.
At least until another character actually give the exposition, I'm still on the fence given Jeanie's records so far.