so i put on my scientist hat and started to write a reply going through the mechanics of beating water's heat capacity, first pointing out that what you want for these types of systems is likely high volumetric heat capacity rather than gravimetric. then i went on to discuss the density of room temp ionic liquids (RTILs), only to find myself digging into their heat capacity numbers and man, you're right! I forgot how well water stores heat compared to other liquids. RTILs vol. heat capacity has trouble breaking 2 J/cm3-K![1]
[1] Table 4, https://www.researchgate.net/publication/231542196_Heat_Capa...