Whether you're stuck on a lifeboat in the middle of the ocean or facing a Castaway-style desert island survival scenario, you'll face the cruel irony of being surrounded by water that you can't drink. This dilemma is one of the reasons surviving at sea is so difficult. With a little ingenuity, you can extract potable water from saltwater, but the improvised methods of doing this all come with substantial drawbacks. For example, a solar still can convert seawater or even urine into drinking water, but it's extremely slow — you might only get a few drops per hour. For maximum efficiency, you'll want a desalinator that strips salt and other dissolved solids from water.
Off-the-shelf desalinators are available, and they can be a great way to efficiently produce clean drinking water from seawater. However, these devices are often large, costly, and/or require frequent maintenance. Creating a DIY unit based on the principle of distillation is a good alternative, and a fun project to undertake.
In the video below, the YouTuber known as electronicsNmore shows off a desalinator he made from a metal water bottle, sterno can, some coiled copper tubing, and an insulated mug. Since distillation relies on quickly cooling hot steam, he also built a solar-powered fan enclosure that constantly pulls cool air over the copper coil.
As mentioned in the video, this design was inspired by a previous water desalinator built by another YouTuber, NightHawkInLight. You can watch that video below for an alternative approach — it's definitely a simpler design that would be easier to build and maintain in the field. However, it requires frequently discarding and replenishing the liquid coolant for the copper coil, rather than using air and sunlight. It's up to you whether you'd want to spend more time up front building the complex (and potentially fragile) solar water desalinator, or spend more time during a survival situation maintaining a water-cooled desalinator.