You can have it at 37 and it will be fine. No different to how warm you have your bath when pregnant. Fill it up and enjoy!
Just turn the temperature down and you’ll be fine. We’ve just done the same and I will be using ours this evening as I’m loving being in the water right now with the weightlessness and the jets on my back. Plus the oxytocin released when you can really relax and zone out will be great for you and baby.
They say the difference is hot tubs maintain the heat whereas a bath starts 37 but then cools, so I don’t know how that works but as long as low enough and not too hot probably be fine but I know it was explained to me that was why the difference. I went away in Jan and there was a hot tub so I sat on the side with legs in as even though it wasn’t that hot I couldn’t tell what would be too hot/ too long
I used a hot tub at around 30 weeks but just for 10 mins at a time and if you feel dizzy or faint just get out straight away. And I kept a bottle of water right next to me to stay hydrated. I think the concern is raising your temperature too much so if you can control it, it's a calculated risk. I had awful back pain and it was definitely worth it, also have lots of baths and don't really see the difference!!!