We shouldn't cut spell themes like that, but I just say we have to consider balance and limitations.
For example, in Oblivion you had to become a master of acrobatics to jump on the surface of water.... and a mage, or anyone who has an apprentice level alteration skill, could cast it as an after thought at level one. Utility spells should definitely stay, but there is difference between, oh say... shooting a fireball and negating the downside of gravity in the game.
Well since there is no more acrobatics skill (I believe it still here just become merged with returned unarmored skill so it perks still here and rogues still have their ability jump from water surface) warriors from their high athletic and endurance can swim fast and at long distance, in Skyrim such things can be handled by stamina and sprinting, what can work in water also.
I can also describe for you better work of waterwalk spell effect before it was
http://www.uesp.net/wiki/Oblivion:Water_Walking
http://www.uesp.net/wiki/Morrowind:Water_Walking
http://www.imperial-library.info/content/daggerfall-spells
Water Walking for D seconds
As we can see there is only one parameter is time under effect, as well there some additional features like cannot be attacked by swimming enemies, still receive damage from dangerous fluids like lava, can move at the same speed as on earth and can do all things thats can be done on earth for duration of spell,
so what can be done
Since we classically have only one parameter thats describe magnitude of water walking its time under effect we will start from this
Water walk can be channeled spell thats consume Magicka in constant rate and can have initial spell cast cost additionally so higher magnitude need more initial Magicka for cast and then still will drain Magicka during spell effect duration, skills in alteration school can affect such costs so skilled mage will have smaller drain of Magicka to continue work of spell, as channeled spell it need to be equipped in one hand until there is not taken high level perk thats will allow use such as passive buffs again, or effect will not come form scroll or potion, as part of Dynamic magic concept there can be concentration checks for maintain such spell under receiving damage or be disturbed by other reason, and spell can dispelled just because mage lose concentration of minds on it so novice can handle buff spell but if will receive an damage spell will automatically dispelled certain battlemage perks can increase concentration of mage, so from what actor can receive damage while use water walk? From water enemies and dangerous environment, water walking is not shield against slaughterfishes how it can save from being bitten but not save from lava?
Its logically thats enemies can reach water walking mage, but he can maneuver and evaded such attacks, not just stay on place.
What next can be done
such spell can be forbidden to cast underwater so need to cast it only while stay on solid objects,
speed of water walking can depend on water type as well as walking speed can depend on earth texture tile under actor simple script will make mud texture actually be mud thats slowing down, as well swamp water will work at the same way also (swift swim effect can protect from this as well as argonian racial trait based on constant swift swim), in such cases eve slippery ice can be done as well.
Spells like burden can cancel effect of water walk and make actor sink as well weigh of equipment can add negative effects to concentration checks for water walk.