You're not the only poster to say that! :3 I totally agree.
I saw in another comment, you were there in 2012? Apparently it's been warming increasingly over the years. Even just dipping my toes in was very uncomfortable, but oh... the temptation to just slide in! Did you see any bathers at the time?
Are you saying it's even HOTTER now? My sister really wanted to get in, and tried, but it was too much. I don't think we saw anyone in the smaller Grjótagjá cave, but if memory serves there was someone bathing in a similar cave nearby - and I think that one was pretty hot as well.
Shortly after 2012, the government actually said, "no more bathing, my dudes" and restricted it (not that they actually do.) I wanted to, and really couldn't get much past a few toes.
Haha, did the sign say "dudes bönnuð"? I think there was concern about unstable rocks on the "ceiling".