Could it be that you're temporarily weakened? What was your magic level at this point?Nope, and my magic level is 99.
Can you check if it works if you take out some water runes? :)I tried it and with the water runes, it show's you can use it.
I think it's an issue with the water staves. If you wear a water staff and you have no water runes in your inventory, it will not show any water spells that you could cast (it still works though)^