[@BCTheEntity]

point taken.  it depends on how far they have gone underground and where this cave is located if it will be semi-warm or not.  I have been to a few caves before. From what i have seen and done, they are always significantly colder than outside because there is no sunlight to warm it up.  
From what i have researched about caves: The only way heat may enter a cave is from overlying rock, underlying rock, air flowing into the cave, and water flowing into the cave.  Seasonal temperature fluctuations above the surface of up to 80°F are reduced to only l°F at a depth of about 50 feet. Considering in some areas the overlying rock is very high above their heads.. this method of heating is pretty much out.  Also, considering [@Major Ursa] mentioned something about the chilly water, and they are clearly below a couple thermoclines i would assume this means water is not a permissible way to heat the caves.  Leaving only one valid method of heating the caves. Since the interior of the earth is hot, technically the temperature of the cave would increase with depth, however this would only be in an area with a fairly high geothermal gradient. We already know that there is water flowing through the cave.  if these streams are flowing from the surface, they would be what regulates the heat or cool in the cave.  and the deepest they could dive safely would be about 200ft, after which it gets dangerous and they need special equipment.  Seeing as none of them were wearing dry suits.. they probably were going to 100 maybe 150ft.  (it gets colder the further down you go, and you can get hypothermia and die if you aren't careful)

Basically there are two types of caves, and this one is looking like its headed for a cold/ice cave.  Though we could have an area like the Crystal Cave of Giants which would act as the cave's core and heat the entire cave?