You can do /thirst to check out your thirst percentage.
The /thirst bar will record your thirst. The higher the bar, the higher your hydration is. If the bar drops to 0, you get dehydrated and will begin to take damage.

Hydration levels will drop naturally over time, making you more thirsty.

Different Thirst Levels
The rate of dropping differs based on the biome and time of day. Players in badlands or deserts during the day will get thirsty quicker, while players in the jungle will get thirsty quicker during the night.
Player hydration levels will not drop during a storm or if they are standing in water. However, you will have a higher chance of getting sick from drinking in the jungle or swamp.
To maintain a healthy life, you will need to regularly drink and have water on you at all times.

How can you drink?

Drinking from a water source such an ocean/river - If you hold an empty wooden bowl in your hand right-click into the water, the player hydration will increase. You can also right-click with an empty hand to drink water.
Drinking from a water bottle - You can bottle up water as you normally would in the game and then drink from that.
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, providing an infinite source of boiled water for the player to drink from.
Using a remedy COMING SOON - Players can use remedies to change their hydration level, these can be furnaces or crafting recipes.
There is a 1% chance that when you drink unboiled water, you will get temporarily sick from it.
Thirst only works in the nations world.