Removing the salt from saltwater (desalination) is usually a difficult, inefficient, and expensive process. However, researchers at Princeton are working on a technique that uses balsa wood as the membrane that strips the salt from the water. This new process promises to use far less energy traditional desalination techniques. New Scientist has a great article with more info. It's not clear to me if this new process will work for small scale applications like ocean survival, but it should be beneficial as an industrial process.
Life raft survival kits often include some kind of tool to desalinate drinking water. Here are three of the more common options.
(image borrowed from Katadyn's site)
Katadyn makes a range of desalination filters, with the Survivor being their smallest. It's a hand-operated pump that removes salt by reverse osmosis. The throughput is just 0.89 liters per hour, with an optimal pump rate of 40 pumps per minute. That's a lot of work for relatively little water, but it will help you while away the hours in the life raft. 5col Survival Supply is a Katadyn dealer, but we don't sell any of their desalinators as of this post.
Pictured above is a Desalter Kit manufactured by the Permutit Company in 1944 for the US military. This kit is on display at the Museum of Applied Arts and Sciences in Australia, and they have a great page with lots more info about it. The short version is that these desalter kits have 8 chemical desalter bars and a treatment bag. You combine the bar with seawater in the treatment bag to produce one pint of drinking water. The entire kit is good for exactly one gallon of fresh water, which doesn't seem like much. A company called Luxfer Magtech still makes these with the product name Van Ben Seawater Desalinization Kit, Mark 2, Type II.
Aquamate Inflatable Solar Still
(image borrowed from Landfall Navigation, which sells the Aquamate still and a range of other excellent SOLAS products)
This inflatable solar still is manufactured by a company called Aquamate Products, Ltd. in the UK. The UK pioneered life raft survival, and they're still at it today. The Aquamate still is an inflatable float that uses solar energy to distill drinking water from seawater at a rate of about 1 pint per day. The great news is that it's entirely passive, so you aren't burning precious calories in exchange for drinking water. Ideally you would have several of these in action, supplying a continuous supply of fresh water. However, the $240 price tag per still might make that difficult.