To make seawater drinkable, you not only have to sterilize it, you also have to remove the salt. Drinking large quantities of seawater can be lethal because of the strain it puts on your organs. Your kidneys have to go into overdrive to filter out the salt, not to mention that water with such a high salt content is never going to rehydrate you. Several ways of separating the water from the salt with basic equipment exist, and one involves boiling the water. You can't just boil the seawater however, you have to boil it and collect the steam -- in other words, distill it.
Place the tumbler in the center of the pan.
Add seawater until it reaches 1 inch below the top of the tumbler.
Place the arrangement on your stove or other heat source and bring the water to a boil.
Reduce the heat until the water is simmering and place the lid on it, upside down. Alternatively, place a wide, shallow bowl over the pan. Place ice in the lid. Steam should condense on the lid or bowl, causing the resulting water to run down to the lowest point, in the center. From there it should drip into the glass.
Remove the glass and empty the distilled water into another container periodically. You can top up the seawater until you have as much drinking water as you need.