Elin,
I measured the container temperatures last summer and found that several things helped to cool the pots ( my repot is here on the forum, you can search for it).
For the terra-cotta pots, the water evaporation through the sides makes the biggest difference, if you water frequently, you are fine. On the other hand, the dry soil in those pots may cook the roots since they cool down very slowly at night.
The plastic pots can be cooled down by covering/shading them with something + watering, even a brown paper bag makes a huge difference.
In the case of SIPs, I am not sure what would play the major cooling role but I do not think the water alone would cool down enough in the extreme heat.
It also depends on the SIP design, wether or not you keep the top open or cover with bark or plastic.
It would be interesting to test.