A Sunken Box is a rare container with valuable rewards that can be retrieved from the bottom of the sea at the Port using the Crane and cleaned at your home base using an Acid Bath. To get a Crane Key, you must complete delivery contracts in the Port building. You need to assemble packages for contracts in the box assembler using certain resources from the packing table recipes.
Contracts are made via the terminal in the central part of the main building. With each completed delivery, your Genesis Reputation will be increasing, and you'll be getting rewards, such as a crane key, parts, extra slots for delivery, and more. The level of your rewards depends on your reputation level. You can view your reputation progress at Jake's in the cargo assembly area.
When you get a crane key, go to the pier and insert it into a special slot in the crane. After that the sunken box will be lifted from the sea bottom.
To deliver it to the base, you need to get to the Port on a chopper and load the box into the trunk. When you return to your base, you need to carry the box to the Acid Bath. You also need to install an Air Filter into the bath and add Acid, which will kickstart the cleaning process. You can open the box and claim your rewards only after the cleaning is complete.

                               
Note! You need to place the Sunken Box into the Acid Bath for cleaning as soon as you bring it to your base. If you don't, in a few hours, the box will turn into a pile of scrap which will attract a horde of zombies, and you will no longer be able to obtain the valuable contents of the box.