There is a chemical based desalination kit available but nothing so simple as a tablet that you can add to seawater and end up with clear, cool fresh water.

The chemical method, by all reports uses a noxious mix of chemicals to cause the salt to be able to be filtered out. And the result is discolored and barely drinkable water. One report I've read from a user claimed that you would have to be desperately thirsty to drink the resulting water.

The kicker is that such kits, because one of the chemicals was a silver compound, were expensive and relatively heavy given the limited amount of water, typically a liter or two, produced.

The almost good news is that there are portable reverse osmosis desalination units available. These units can produce many gallons of good quality drinkable water. The down side is they are expensive, last I looked the smallest unit was about $600, and require a fair bit of time and physical effort to work.

Dying of thirst while surrounded by water has been the bane of sailors through time and stranded sailors would happily trade a fortune for fresh water when they had none. These units are reliable and capable of producing enough water out of seawater to keep several people alive indefinitely and are operable by nearly anyone. The US navy has taken to putting them in life raft survival kits.