Didn't make as much progress as I would have liked today, but I got the UI and magicka bar bits of the magic system working at least. The magicka bar will deplete as you cast spells and regenerate a little each 'turn' when you are not casting a spell (which at the moment means every time you make a move). If you don't have enough magicka, you can't cast the spell (as you would expect!).
You can also select the two spells other than Water Jet but this won't actually do anything visible yet. Hoping to get Ice Wall and Water Walk implemented tomorrow so I can move on to enemies. (And also, at some point, make walls be solid.)