It may not be relevant in too many places, but…
I'm heading to Iceland again this year and I will carry a colourful jarðfræðikort (geologic map) with me:
(available online here).
The topographic map does not list hot springs. The tourist map only lists some of them, in particular the ones with tourist infrastructure. But the ones that are deep in the wilderness, tens of kilometres from the nearest frequented road, may only be indicated on the geological map. On the map I have, the hot springs are categorised by temperature, so I know that the boiling ones may be fun to watch but not so fun to bathe in. But imagine being deep in the wilderness, it's 5°C, raining, and windy, and then arriving at a hot spring at 35°C, all for oneself (because no-one else brought a geologic map!). Hmm.....!
In a geologically infant area such as Iceland, the age of the land may also be relevant. 1000 year old land is quite different from 100.000 year old land. But that information can be more directly obtained from a vegetation map where available.
Edit: Finally, it was less useful than I hoped. Firstly, the scale is not nearly good enough for outdoor use, and I was unable to find some of the hotsprings listed. Secondly, a hotspring is not yet a hotpool. A bit of hot water coming out of the muddy ground is a nice curiosity. But only when someone has built a pool around it does it become an extraordinary backcountry luxury. The map is a good starting point for the latter, but it's not quite enough.