Why did people who were shipwrecked often die of thirst, although there was a lot of water around?

Because seawater is not suitable for ingestion. There is a chance of catching an infection, it is also impossible to get drunk with salt water (it is salty after all). So when you find yourself alive after a shipwreck, try to make a fire and put the saucer on, for example, start boiling sea water, the steam turned into water droplets (you need to put something over the saucer) is already drinkable.