Thirst
When enabled in the config file, thirst will place a bar at the top of the players screen when they log in. This bar will record the players thirst, the higher the bar, the higher the players hydration is. If the bar drops to 0, the player is dehydrated and will begin to take damage.
Player hydration levels will drop naturally over time, making the player more thirsty. The rate of dropping could differ based on the biome and time of day. Players in badlands or desert during the day will get thirsty quicker, while players in the jungle will get thirsty quicker during the night. Players will always get thirsty more quickly in the nether.
Player hydration levels will not drop during a storm or if they are standing in water.
Players will have a higher chance of getting sick from drinking in the jungle or swamp.
You can disable thirst completely in the config, or you can prevent it in particular worlds.
You can increase your hydration in a number of ways:
Drinking from a water source such a ocean/river - If you hold an empty wooden bowl in your hand right click into the water, the player hydration will increase. However, this is NOT considered a safe method of drinking and could cause the player to become violently sick.
Drinking from a water bottle - The player can bottle up water as they normally would in the game and the player can drink from that, which will increase the players hydration, however, this could make the player sick as if they drank directly from the water.
Drinking from a cauldron - Players can drink directly from a cauldron of water, though it can still cause the dirty water sickness. This can be negated by placing a campfire directly below the cauldron, provides an infinite source of boiled water for the player to drink from.
Using a remedy - Players can use remedies to change the players hydration level, these can be furnace or crafting recipes. Boiled water and snow-melt water are included by default.