I just tried out the Water Walking spell, and it's really great. In fact, I think it may be slightly overpowered. My Archmage can stand in the ocean and shower coastal or inland cities with meteors, and if they don't have any boats nearby, there is nothing they can do about it. He can even reach inland cities which aren't able to built boats.
Some suggestions for weakening the spell a bit, ranked by severity:
1. A mage cannot cast Fire spells while standing on water. Makes sense in an odd sort of way, and prevents Meteor Shower, which is probably the most potent of the offensive spells. He would still be able to use all other spells from the safe distance, however.
2. A mage can ONLY cast water spells while standing on water. So he can basically just use Tsunami.
3. A mage cannot cast spells on water.
(Note: Even with one of the above restrictions, it will most likely still be possible to abuse the spell by using 1 move point to step inland, cast a spell, and then step into the sea again. Perhaps a mage loses all movement in the transition between water and land?)
4. Water walking is not permanent. It has a 5% chance to break each time a mage ends his turn standing on water. With this, none of the above restrictions would be needed, but it might be frustrating to risk that one's experienced Archmage drowned.
I also tested the Tsunami spell - it doesn't seem to do anything. I tried using it against a spearman, an improvement and an empty square, no effect in either of the cases.