Water magic sucks in combat except for falling frost, frozen heart, quickness and the various underwater spells which are all sheer gravy (except, okay, the zap spell, that's not much good) - in other words, it sucks except for all the spells it gets, which are good
Blood Rain is often an outstanding spell, especially if you have a bunch of fearless units.
Agony is also more effective than it looks. With the great range and large AoE, you can rout whole armies with it. Falling foo is better but that doesn't mean Agony isn't good.
Blood Vengeance is actually worth casting on a number of units - if you head straight for Blood 9, you can have Jotun Skratti cast it and then turn into werewolves. Instant SCs - this really works, although anything requiring a 9 that just clears chaff is, admittedly, not infinitely exciting.
Might as well cast Pain Transfer at the same time if you're swimming in slaves.
Blood Lust is always worth casting as Lanka.