Water Ball is a level one spell from the Book of Seas, which is the prerequisite for Maelstrom. That said, I've never seen anybody use Water Ball, and it baffles me. It's a seriously useful spell! Anyways, let's dive into this:
It's easy to aim, and most balls really aren't. For example, Shining Bolt lofts across the screen uselessly, Ice Ball doesn't go anywhere, and Fireball does too straight. Water Ball goes exactly where you tell it to go.
6) Decent damage.
It does 60 damage, and while that isn't exactly game-breaking, it's not too shabby. It's a little better than Fireball, and more useful, to boot.
You need it if you plan on using the supremely useful Maelstrom. I don't think elaboration is needed.
4) Splash Damage.
It's more than possible you'll hit nearby enemies- it's almost certain. Use it when the enemy bunches up, and suddenly they're all hit for decent damage.
3) Bunker Busting.
Used at full strength, it makes a total mess of anybody who's tried using the terrain to hide. Slam one into their hidey-hole, and you can stop by and say hello in person.
Use it at full power to escape from a mud ball/vine bomb, and hurt the idiot who used it on you, too. It takes back your momentum, and take the offensive again. Be careful with this, though, as many people carry shock bombs, or they'll just slap another one on you, again. Of course, since most entrapping spells do poor damage, you can very likely out-damage his poor, fool self.
1) Pimp Factor.
Everybody uses the same mud-ball -> shock bomb -> lightning bolt -> mega boulder combo. Do something interesting and original, and surprise your opponents for once!