I visited the Blue Lagoon a few years back. It had no impact on my levels.
The lagoon is just like a big warm outdoor bath. Not really hot enough to have any effect.
I also see little change in a sauna but I am more cautious because it could be difficult to feel the symptoms of a hypo in the heat. So I recommend keeping an extra close eye on levels before going in.
@Taffygirl If your daughter uses a Libre, I find they do not like the humid heat of a sauna. For me, the affect is temporary: it returns to normal after a couple of hours. In the meantime, you need to be extra vigilant to highs or lows with finger pricks because my sensor readings became like a random number generator until it stabilised again.
I am really glad to read your daughter is "living her life again." For me it has been very important to my mental health to make diabetes work around me and my life rather than the other way. I have not found anything I want to do but diabetes stops me. And I promise I stretch the envelop a lot! Iceland was one of my least adventurous/active/envelop stretching holidays