let's have a pool party!!!!!
technically a hot spring party (meh)
and the (technical) difference would be.....
Hot springs:
- use hot water
- are relatively shallow
- expect you not to fool around in them (at least not too much)
- are meant for relaxation and physical hygiene
- used in the nude (generally, only mixed gender onsen will allow clothing like bathing suits to be worn while using them. Otherwise, you are expected to be naked with other patrons in single-gender baths)