I have done a bunch of research and am pretty passionate about this subject. Your midwife's receptionist is correct, as is plantmommy. The problem with hot water is the risk of increasing your body temperature, above 102 for a prolonged period of time, the same reason it is dangerous to get a fever while pg. First of all, it is most dangerous in your first tri as it is associated with m/c. However damage can still be done later in pg, but if you use your head, you can enjoy hot soaks without worry.
You need to know that it takes time for your body temp to go up. There are a lot of homeostasis measures your body will use to counteract heat, and one of the first things you will see is flushing/redness in the face. Your temp is increasing very slowly (fractions of a degree) when you begin to sweat, which typically happens AFTER you get red in the face. I sit in a 104 tub regularly (I'm 30 weeks), and am a very heat sensitive person to begin with, usually after about 10-15 min I will start to flush, and by that time I am usually ready to get out. I have stayed in until I begin to sweat, but normally have my fill by the time I'm red. But even sweating for 10 min will not increase your body temp enough to get to the danger zone.
So, after all that, to answer your question, you can stay in water below body temp for as long as you please, it cannot increase your temp, so you have no worries. Enjoy!