I know affinity points increase your chance to improve spells AND decrease the amount of mana those spells will cost. I'm not sure if they affect the damage or chance for opponents to save against.

I don't know why anyone would view water as less essential due to the fact that shield of waves stops CRANIAL. My guess is if anyone dropped water it's because they knew they wouldn't be facing many mace specs. And shield of earth stops bearcharge, so maybe if you're going outlander you need it as bad, but again all paths are awesome and I personally would never drop one except for a role idea.