I know most people will never be on a boat that sinks and be in the situation, whereas they would have to try and survive by drinking saltwater. First, don’t drink saltwater it will kill you. But there might come a time that you are on are out in the field for a long time, and you have to find water, and its either get it from the ocean or possible from a river or swap that has brackish water (half salt/ half fresh). The best method is to have a pump with you that can be used for saltwater. It will be a lot of work pumping saltwater and turning it into fresh is a lot of work for a little reward. There is also a way to get clean water out of saltwater that uses a leaching method that you can fill and forget. HTI uses Osmosis is the natural diffusion of water through a semi-permeable membrane from a solution containing a low concentration of dissolved solids to a solution having a higher level of dissolved solids. When it comes to the best method for you that depends on the situation, I am going to talk about a few different techniques and also suggest a few things you can just buy and keep around.
Getting salt out of seawater requires the following essential components. It doesn’t matter what you do to accomplish this, but they should be as clean as posable.
1. Method of evaporation
2. Something to catch condensation
3. A way to collect the condensation back into a substantial container.
There are a lot of things you can use to accomplish this, and during a real survival situation, knowing the necessary actions will go a long way. You should always carry the right items, so if you find yourself in this situation, it will make it a lot easier. All of this will go a long way and could save your life. You should always have items to purify water on your boat because you never know what will happen. You can have this in your boat, and if you need it, it is there.
Above is a basic Solar still, you can even just put a plastic bag on green vegetation and collect water that way. 
This can be accomplished with a poncho also. Beach well. Along the coast, obtain water by digging a beach well. If you are near a beach you can go back on the beach or inland a little bit and dig a beach well, let it fill with water and drain it at least three times before you drink it. It should be boiled or treated.
There was an 18year old kid from Indonesia that survived 49 days on a tiny fishing boat, and he used his clothes to filter the water thru to make it (more) drinkable. He used his cotton shirt and drank the water thru it. Some studies have shown that filtering water through a sari, is a garment that is commonly worn by women in the Indian subcontinent, can significantly increase its potability. In 2003, scientists discovered that filtering water from rivers and ponds in Bangladesh through a folded piece of cotton cloth taken from a sari cut the risk of infection with cholera by half. Interestingly, they noted that old fabric makes for a better filter than new material because the pore size of loose threads is smaller.
In a follow-up study in 2015, researchers found that a filter made of four layers of worn cotton material could filter out more than 99 percent of all cholera bacteria.
One of the biggest things to remember in a survival situation is, do not eat if you don’t have water. If you have water available to you, you should try and filter it as much as possible. But if it comes to, I am going to die if I don’t drink the water. Well, most survival experts say to drink it. (that only counts for freshwater not water with salt in it) When you are rescued, a lot of what you can get from drinking bad water can be treated.
