To complete this Side Easter Egg you will need to collect three items from around the map.

1. Crown

The first item you are looking for is the crown. This is located at the top of one of the ships large chimney stacks on the Bridge. You can shoot it with any weapon but you'll need to do it a couple of times as it will fall down to a lower ledge and then it will drop to a bench. Interact with it to pick it up.

2. Moustache

Next up is the moustache which is located on the Promenade Starboard Deck hidden in amongst the vines at the top opposite the fan.

3. Trident

The third and final piece you'll need to get is the trident. This is extremely well hidden down in the Engine Room up high in the rafters above the Odin perk machine. It's located on the side of a metal girder and is extremely hard to see so you can either throw an explosive up there or better still use a sniper with a good scope to be able to see it.

Now that you have all three items you'll need to head down to the Galley and interact with the zombie stuck in the wall. You'll place all three items on the zombie.

After you place the items you'll hear a zombie screaming noise indicating the easter egg has activated. The next round of zombies will now have blue eyes and will slowly walk as if they are under water.

You can repeat this every 9 rounds by simply interacting with the zombie and you'll know it's worked as you will hear the scream again. If you hear a clicking noise it just means you have to push the rounds a little more.