If you've already unlocked it the Hot Spring is found in the lower corner of the area where Galen and Nina live.
If you haven't unlocked it yet there are two ways to do it:
Either get Flora to 250FP and enter the area with the waterfall between 11:20 pm and 12:00 am on a sunny day (it can't be Sunday).
Or if Flora is married the event won't happen so you'll need to get married and have a baby. After that the Hot Spring is unlocked automatically. No scene happens it just appears.
