I don't remember if this is a LN or manga A Japanese guy gets transported to a parallel world that have magic. So his high school becomes a Magic high school. His life is the same (or very close). And some water spirit girl lives with him. It's kinda vague but hope someone know it : )
Sounds like versatile mage but he's not Japanese and doesn't have a water spirit living with him, as far as I've read. Sorry for not being much help, don't really read JP novels.