I'm surprised no one has commented on this yet. I really like the idea of an elemental water caster (we already have ice, fire, earth, nature, darkness, light) and he looks pretty fun to play. I like the creativity of the passive, but I feel like it wouldn't compliment your waterfall wards very well. It would require the champion to be casting spells when someone hits the waterfall in order for it to slow, which is unlikely unless used mid combat. I think your W needs a rework. I would suggest getting rid of the ward function and replace it with something more combat effective, like healing teammates who stand under it, decaying the waterfall.

Thanks to Jovy for the sig!
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Upon casting a skill, Dylan gains a stack of Tide for 5 seconds. Having stacks of Tide will enhance the effects of his skills. Upon achieving 5 stacks, the stacks are reset and Dylan gains a shield with 200 (level 1)/350 (level 7)/500 (level 13) health for 7 seconds.
Q: Chain of Streams
Dylan deals 60/100/140/180/220 (+0.5) magic damage to the target enemy champion and restores nearby allied champions’ HP by 1/3 of that amount + 50 for each stack of Tide Dylan currently has.
Cooldown: 9/8/7/6/5 seconds
Mana Cost: 50/60/70/80/90
W: Violent Waterfall
Dylan creates a waterfall that stays on and reveals the target area for up to 1 minute. The waterfall deals 20/32/44/56/68 (+0.2) magic damage per second to units that stay on it, and decays every time it deals damage. If the waterfall decays 5 times, it will disappear. The waterfall will also slow targets in the area by 15% for each stack of Tide Dylan currently has. Dylan can place up to 5 waterfalls at the same time.
Cooldown: 20/18/16/14/12 seconds
Mana Cost: 60/70/80/90/100
E: Water Hand Prison
Dylan summons a targetable elemental hand that holds his target enemy champion in place for up to 2.5/3/3.5/4/4.5 seconds. If an enemy champion attacks the elemental hand, it disappears and the target is freed. Each stack of Tide allows the elemental hand to survive 1 more hit.
Cooldown: 11/10/9/8/7 seconds
Mana Cost: 70/80/90/100/110
R: Turbine Wave
Dylan transforms into a wave and moves swiftly toward the target location, dealing 150/250/400 (+0.8) magic damage to units on the way and knocking them back with him, toward his target location.
Cooldown: 150/135/120 seconds
Mana Cost: 100/125/150