FWIW,
When I had a NASTY sinus infection last year, the Doctor actually told me to get into the spa (with some Eucalyptus) and that would help to loosen up some of the congestion.
I questioned going in with a fever - 101. He stated that the water temp at 101 would not drive it up - so not to worry.
AS with all things - follow your own doctor's advice. BTW - it made me feel better.