If we fall into a sea of liquid, how hot does it need to be for us to die instantly and painlessly?

Instantly is a matter of opinion, so let’s focus on painless.

Temperature is tricky as you will have some build up prior to the point of execution, also orientation and speed on emersion can become significate. So let’s layout the most likely scenarios.

• Destruction of the body faster than nerves can fire and be processed by the brain. 15 thousandths of a second is about right for nerve signal and about three times that for processing. Regardless I would think a second or less for the brain to be destroyed would likely satisfy the requirement.
• Loss of conscience faster than nerves can fire and be processed by the brain followed by the destruction of the body before waking up. Again, loss of conscience within a second would seem to satisfy the requirements with the rest occurring on a slower schedule.