You need 3-4 players...
Credit goes to L0NESOLDIER93, Gordonfreemaned, Stevelongbone...

Part 1 - Powering the door - The door is located in the very bottom of the lighthouse, to get there you can take the stairs located to the right of the main lighthouse entrance, follow this down and drop down, you will find a steal/vault door. At the moment nothing will happen if you came straight here this is because the power isnt on. So the first part is working on turning the Power on. The power is located on the ships top level and is required to open 4 doors (750+1250+750) this is for the first boat bloackade then the stair blockade and finally the ships top deck locked doors. Once here turn the power on ... if done correctly your screen should flash, now head to the vault door which was mentioned earlier.

Part 2 - Power cell - From turning on the power head over to the vault door located under the lighthouse, when getting to the door. you should some people talk, (people believe its the characters from the other zombie maps) after this dialog look around this room or the room above for the power cell, you need to find this and put this into the circuit breaker box right of the vault door. If done correctly proceed to part 3.

Part 3 - The generators - after putting the cell into the box, the door will have a red glow around it and you will hear a voice tellign you, that you need to destroy 4 generators, one is located right behind you, the 2nd one is located outside of the map near some rocks towards the right side of the map...(these are easy to spot from the ship deck due to giant red glow) The 3rd generator is located on the right side fo the ship, once opening the first blockade to enter the ship area go up the stairs and turn right, buy that blockade i think its 1000 or 750, from here go left and look off the ship edge (looking into the sea) around the side you will see a dim red light on a small platform, cook a grenade and destroy it..theres the 3rd generator done, the fourth can be spotted from the platform you walk on to get into the vault room, go down the stairs and go into the corner, from here look foward for the red glow. (this is right next to the door)
Once completing this task in destroying the generators head back to the vault.
NOTE: confirmation that Grenade DOES work, Also the Crossbow DOES with 3 shots, SEMTEX DOES work, MW2 LAW DOES NOT work!

Part 4 - Finding the Vodka - once completing the task with the generators you wil be told to find some Vodka. These are the present locations of the Vodka:
Vodka - It's encased in ice with the bottle sticking out of the top and it's ALWAYS hanging off a railing where a team mate can be below it to catch it.
-First place was right where the two boats connect (power switch boat and jugger boat) where you buy the debris which drops you below onto the jugger boat. It was on the left railing.
-Second place was where the fuse box is, go up to where the mystery box is and it's on the railing right there over hanging the section with quick revive.
-Last place was on top of the light house over hanging the section where you zipwire over. You have to knife it and it drops and someone HAS to catch it, if you don't it will spawn elsewhere.
Once finding the bottle, the person he is CARRYING it must goto to the door, next to the door (left side) there is a tube, press X and it will suck the bottle up moving to Step 5

NOTE: these bottles can spawn in any location around the map aslong theres a rail and a place below to stand.
NOTE:This part i do NOT think is Required in solo since i do not understand how to get this part completed.
NOTE: If your playing in SOLO i believe you skip this part or you do something else but im sure you skip this part.

Once completing this task head back to the Vault.

Part 5 - The Golden Rod - Right to Do this, you need to do many things firstly goto the ship's top deck and move the steering wheel twice (5 o'clock) then to there left theres Many leavers, the left one needs to be moved Once, you DO NOT touch the middle one, then press the right leaver 3 times. This will make a submarine appear HOWEVER! unlike SOLO the green beam from the Lighthouse WILL NOT appear this meaning you need to do another task to activate the beam, My team has worked out you need to Find 4 special Radio's which transmit a Morrse-code (sounds fishy but honestly true) with these 4 radio signals give the morse code (From now is my mates theorey however the above solution for Part 5 is working and confirmed so far) he says, from having this code you need to basically put this code in somewhere or something then the green beam appears, from the green beam appearing you do the EXACT same as the SOLO, you fire the VR-11 at a zombie to make it humanize then follow them into the green beam, shoot them and the golden rod appears.

From here run to the vault and use the rod on the tubes located to the left the rod should go up, after the some dialog the fuse should blow, knife the fuse/door for more dialog then the achievement should unlock.

Combination Guide:
With the numbers I have found that would make the following code from the top of the lighthouse down:
Top Floor Yellow - 2
Second Floor Orange - 7
First Floor Blue - 4
Bottom Floor Purple - 6

NOTE: THE CODE DOES NOT CHANGE BETWEEN SOLO OR CO-OP. HOWEVER THE NUMBERS EACH DIAL STARTS ON DOES.

Now you could just input 0-9 for blue and figure it out, but this isn't the case because when you move one dial others are "attached" to it.

So far this is what I've figure out with regards to that:
Yellow moves 1 = Orange moves 1
Orange moves 1 = Yellow moves 1 and Blue moves 1
Blue moves 1 = Orange moves 1 and Purple moves 1
Purple moves 1 = Blue moves 1

Basically and dial you move will move any dial around it. Making it hard to just guess a number. aa) 3333ch 098))