Water Park: Easter Egg

Before starting

The main quest of Water Park is designed by FreeAccount and Glitch, with the boss fight scripted by Planet, according to the Workshop page, and it can be played in solo or co-op. It brings four batteries to the boss portal, the portal machine next to the teleporter, and each battery is followed by a challenge of the robot N.3.1.L. The order below is the one of Sassy Asi, which matches YaBoyBadger, the Steam text guide of CustomZombieKiller and the Notion guide of rudyspg.

YaBoyBadger advises a Pack-a-Punched weapon, the VPR in that guide, and the five best perks before the escort of David Hasselhoff. Several players report bugs on the Workshop page: N.3.1.L not giving the next challenge, clown rounds that never end, and crashes.

Step 1: power and Pack-a-Punch

Build the power at the shack of the beach with its three parts.
Turn the four valves and link the teleporter with the button near Tough Enough.
Teleport to the Pack-a-Punch room and take the head of N.3.1.L from the barrels beside the machine.
Back in the park, put the head on the robot’s body, by the teleporter, and talk to it: it asks for the red battery.

The parts, the valves and the teleporter are listed on the pages of the buildables and of the Pack-a-Punch.

Step 2: the red battery

Enter the three colored numbers of the clipboards in their panels: the gate of the Pack-a-Punch room opens.
Take the dynamite inside the gate. It can only be taken once N.3.1.L has asked for the red battery.
Place it on the metal door of Cone Lord, near a valve. The blast frees David Hasselhoff.
Talk to him to start the escort. He walks slowly to the beach while zombies spawn without end; a player who strays too far from him takes damage.
On the beach, he walks into the water and sends back the red battery. Place it in the boss portal.

Cone Lord

The ice cream stand of Cone Lord, its giant cone on the sign, the area of the door of David Hasselhoff.

A dark metal door between two striped pillars in a red brick wall, with the prompt Press to place, Water Park, Call of Duty Black Ops III Zombies

The metal door

The dark metal door between two pillars, with the prompt to place the dynamite.

YaBoyBadger took a jump pad by mistake during the escort and came back in time, so slow is the walk. Two players on the Workshop page warn not to start the escort during a clown round: the exploding clowns kill David Hasselhoff.

Step 3: the challenges of N.3.1.L

After each battery, N.3.1.L gives a random kill challenge, shown by a progress circle on the left of the screen. The Notion guide of rudyspg gives the counts:

Challenge	Kills
Kills	100
Melee kills	40
Grenade kills	5
Headshots	50
Kills while crouching	45
Kills while prone	60
Kills while in water	100
Kills of zombies in water	100

Each challenge ends with a coin from N.3.1.L, to put into a souvenir machine. For the grenade kills, a Reddit thread says that thermite does not count, but monkey bombs and a grenade launcher do.

Step 4: the green battery

After the first challenge, take the coin, then talk to N.3.1.L again: it gives a pair of yellow sunglasses. On a controller, they go on with the up button of the d-pad; Sassy Asi warns that the icon of the HUD is wrong.
With the glasses on, find the colored arrows on a wall. Sassy Asi has seen them on the wall of the spawn, left of the Hydro Paradise sign, or on the gate under the Journey into Space building, near Kraken’s Revenge.
Enter the code on the arcade machines of the ice area, near the blue slide, in the order of the arrows.
A meteor falls and breaks a small box across from the machines: take the green battery and place it in the boss portal.

The two guides read the code in different ways: Sassy Asi matches the colors of the arrows to the colors of the machines, YaBoyBadger the directions, the left machine for left, the middle one for down, the right one for right. The code changes with each game.

The Journey into Space building with cyan, purple and pink arrows on its shutter, a Magic Wheel in front and a giant rubber duck beside it, Water Park, Call of Duty Black Ops III Zombies

The arrows

The colored arrows on the shutter under the Journey into Space sign, seen with the glasses on.

Three colored arcade machines, purple, red and blue, with the prompt Press to place, Water Park, Call of Duty Black Ops III Zombies

The arcade machines

The colored arcade machines where the code is entered, with the prompt of the game.

Step 5: the blue battery

After the second challenge, take the coin.
On the beach, shoot the X on the lighthouse; Sassy Asi uses a Pack-a-Punched weapon. Its beam lights the place of a box.
Go to the souvenir machine near Racin’ Stripes and knife the burning box beside it: it drops a key.
In the cave of Skull Bay, open the cage of the skeleton with the key. A lockdown of about 45 seconds to a minute starts, with skeleton zombies.
When the cage falls, take the blue battery and place it in the boss portal.

Sassy Asi stands in a corner of the cave where the skeletons come from a single side.

Step 6: the yellow battery

After the third challenge, take the last coin and the bucket of N.3.1.L.
Make a water gun: the three coins in a souvenir machine, then the bucket filled with coolant at the beach, carried to the machine before it overheats.
In the Journey into Space area, near the rocket, shoot the yellow battery out of the ice arch with the water gun.
Climb to Quickies and ride the slide down: the battery is picked up at its end.
Place it in the boss portal.

The four water guns and their routes are on the page of the weapons.

Step 7: the souls of the portal

With the four batteries, the portal says its power is low. Kill zombies near it until their souls fill it; YaBoyBadger does it with the Dischord. Once full, the portal leads to the boss fight. Before going, N.3.1.L gives a last challenge for a free perk, according to Sassy Asi, and the shield and the free perk of the candy boxes can be useful.

The boss fight

The portal sends the players to the beach, closed off, against a giant creature and zombies that keep coming. The fight has three phases:

Put on the glasses to see the weak points of the boss, four for Sassy Asi, and shoot them all. The glasses do not stay on, and a hit from a zombie removes them.
The boss attacks with a green laser, which goes through the bamboo hut, for about a minute. Then the glasses and the weak points again.
The boss slams the ground and sends falling rocks, marked by bright orange spots, for about a minute. Then the last weak points: the boss falls apart and the zombies stop.

YaBoyBadger advises not to kill the zombies of the arena, which do not come back in great numbers if they are kept in a horde. One player on the Workshop page says the boss has no weak points in some games.

Rewards

Perkaholic

Every perk, the reward of the boss fight.

The buyable ending

Once the boss is beaten, the boss portal ends the game for 60,000 points, according to Sassy Asi. The text guide of CustomZombieKiller also places the ending at the boss portal.