I find Water is a bit of an unusual one, the first 3 tiers don't feel very combat orientated, it's combat spells tend to just be disruptive - however when you get to tier 4 and 5 it really brings the pain with smashy face spells on par with fire. Not a bad set of spells overall IMO
Meditation - this used to be a must-have for me in earlier betas although not so much now due to the essence slots restriction - I still use it though.
Slow - a mild debuff, pretty handy if it sticks
Freeze is amazing, a very powerful effect that can really help when you're on the defensive - devs please don't nerf this
Chaos used to bypass resistance checks but now it's subject to them and consequently I find it less appealing - you used to get a random effect but you knew it would stick - I still use it occasionally but not so much
Pandemonium - this is a nice cheap spell - OK it's random effects and now subject to resistance checks but it's also only 12 mana so I find it's often worth a punt.
Spell leach - this is my favourite counter spell because you can really milk it to get lots of mana - all you need to do is find a monster that casts spells and counter his spells with this a few times - you can wrack up loads of mana this way
Summon ice elemental - I like the change to ice elementals (frostborn) and if you go with a mage build plus mantle of oceans then this little guy only costs 7 mana as a tactical summon, which is pretty good IMO, especially since he'll also debuff what he hits and they'll waste a couple of turns hitting him
Blizzard - even with a 2 turn casting time this spell is still excellent - it does tonnes of damage and only costs 30 mana and even with 2 turns to spread out you're still going to get a bunch of units with this
Tidal wave - this looks very cool but it's hard to assess how good it is, it appears to do pretty low damage but it hits all units in multiple armies and is fairly cheap too - it's very situational but has merit
OMG water gets some really mean tier 5 spells - Vetrar's howl annihilates multiple armies, it's a very strong spell that deserves to cost so much mana. Grip of winter is also really powerful too, halting all enemy production is so crippling - I used this in earlier betas but not recently. I think disenchant works on it but I've never had a chance to test this out. Does anyone know more about this? One thing I want to know is whether a player needs to cast disenchant once or separately for each city (ouch)
Water gets an interesting and useful set of spells IMO. They should put mantle of oceans back in as exclusively water (not water2life2) and nerf it too.