Here's a couple more photos I took while 'testing' to see how significant the cooling effect was.
Here we have one empty terracotta pot, one full terracotta pot (it's darker as the water has seeped through the walls) and one plastic water bottle. I'd filled them with water two days previously to give the temperatures time to settle down. The water in the plastic bottle is 23.2 C and in the terracotta it is 20.4 C. Not a huge difference, but every little helps!
A couple of days later I thought I
should measure the temperature of the surface of the pots. It was a hotter day, so the temperatures don't really relate to the ones in the previous photo. I stuck one probe on the surface of the empty pot - 27.5 C, and the full pot (jammed in the crook of the handle as it kept falling off because of the wet surface) measured 22.7 C. I'm pretty sure that it was acting to cool the room as much as the water.