Jains actually do not drink hot (garam) water with living organisms in it. What is usually meant is:

• Boiled, then cooled water (ukāḷyuṁ pāṇī), not continuously hot water.
• And it is almost always filtered as well.

Now to your main doubt:

> “If Jains believe in ahimsa, isn’t boiling water killing water organisms?”

1. Two options, which has less himsa?

• Raw / unboiled water

Contains countless live microorganisms. When you drink it, many are crushed, digested, destroyed inside the body.

• Boiled & filtered water

Many organisms are destroyed at the time of boiling, then the dead bodies are removed by filtering. The number of jīvas harmed while actually drinking is far less.

Jain dharma teaches: “Jitni himsa tal sakti ho, utni talni chahiye.” “Reduce violence as much as possible.”

So, even though boiling involves some himsa,

• drinking unboiled water = continuous, massive himsa every time you drink
• drinking boiled & filtered water = much less himsa overall

Therefore, for monks and strict followers, boiled, filtered, and time‑limited water is considered the lesser violence and thus preferred.

You can read more here

2. Why “hot” (garam) water is mentioned

In common language people say “garam pani”, but in practice:

• Water is boiled to kill/inactivate many micro‑organisms.
• Then cooled to a drinkable temperature.
• Often must be used within a certain time (e.g., within a few hours or before sunset), because new life can arise again if it is kept too long.

So the Jain aim is careful, limited use, not casually drinking hot water that is full of organisms.

3. Intent (bhāv) also matters

In Jainism, intention is very important:

• The purpose of boiling is not “I want to kill germs,”

but “I must drink to survive; let me choose the way that harms the least number of jīvas.”

• This is similar to walking: every step kills tiny beings in dust, yet walking carefully is still allowed because the body must function.

Thus, boiled water is a practical compromise:

• Life has to be sustained.
• Among possible ways, choose the way that reduces violence the most.

In summary: Jains drink boiled (then cooled and filtered) water to minimize, not increase, the killing of water organisms.