[Fiberwyre_P2P]

I would make the participation prize 5 ECs. Being a zombie sucks when players are moving at 2.0 speed after killing 10 zombies. I would just leave at that point if I have no chance to win the zombie prize.
1 EC for at the very least a 15 minute event isn't that great. (or maybe base the reward they get off how many kills they get as a zombie.)

Also, instead of randomly choosing someone to be the King zombie, you could have it based on skill (or volunteerism).
Everyone starts off in a room where waves of zombies (NPCs) pour in, and the first one to die (or sacrifice themself) becomes the king.
obviously once a king is chosen, you just warp everyone back out to the map and resume the event as usual.

And maybe let people become recruited to the military after a certain number of zombie kills?

And one more thing. I hope you spend extra time considering how to balance against people just hiding in the water.
My suggestions on how to deal with it would be:
a) let zombies melee in water (by itself wouldn't solve the problem, as the water is a vast expanse of empty space where players can easily evade zombies)
b) make the water infected (damages players over time)
c) zombie sharks (attack players as if they're bleeding whether they are or not)
d) slow survivors significantly when they're swimming.

but despite all the pro-zombie stuff I suggested, don't make them too overpowered.