[Sandman]

Some more water spells:

Discipline

Grants friendly units in a square +4 defence. Basically a water variant of protection. A costly 'mass discipline' could be available as well. This could a rather powerful spell, so an associated pay-off (like protection's fire vulnerability) may be in order. It's a mind spell, so it won't affect mindless units.

Weakness

Enemy units become weaker as their muscles soften and their joints sieze up. Reduces strength by 4 points. Magic resistance negates. Available in single square, small area and large area variants.

Memory Focus

This enchantment opens and nourishes the casters mind. His memory becomes as clear and malleable as water itself. The caster gains greatly increased experience points from each battle, perhaps triple or quadruple. There could also be an expensive army-affecting Version of this spell, or even a ritual which grants this ability to all your units everywhere in the world. The ritual spell would be a very nice research booster as well.

Amnesia

The opposite of memory focus, this spell clouds the memories of enemy units. When hit, their experience is washed away and a unit of battle-hardened veterans becomes as inexperienced as raw recruits - but with several battles worth of afflictions. Magic resistance negates, and mindless units are immune. There could also be a ritual spell which causes experience to wash away across the whole world; except for your soldiers. Soon, you'd own the hall of fame.

As well as these memory affecting spells, there could also be water items with related effects. An item that grants permanent memory focus, and a sword that drains enemy experience, for example. That'd be quite a combination...

Summon Sirens

I'd envisage sirens as a sort of aquatic combination of hama dryads and wailing ladies. Aquatic, awe, ethereal (maybe), steal strength and an area affecting siren song which could have all sorts of interesting effects. They can be summoned anywhere. It's similar to sea nymphs, but I reckon sirens are cooler.

Sea Witch's Coven

Summons a sea witch and her coven of thirteen sirens. The sea witch has powers of water and death, and her song is especially powerful. When in a water province, she can summon lost sailors to her cause. The lost sailors are poor amphibians, and have almost lost their minds, but still have some combat skills.

Awaken Aboleth

A level 9 water conjuration. Hugely powerful. Must be cast underwater.