If you have a bath unit or a chiller (such as a Minichiller), then you can potentially use water as thermal fluid.
Note that usually a mixture of water and ethylene glycol is used rather than just water, to widen the temperature range – adding glycol reduces the freezing point. (Search these FAQs for ‘glycol’ to find the entry ‘Do I dilute ethylene glycol for use in a circulator? By how much?’ for further details.)
Please be advised that if you have a Huber Unistat, you should not use water or water-glycol as thermal fluid – select a suitable silicone oil for the temperature range instead.