While cooking, I have noticed that a ball of boiled spinach retains its heat for a long time. I understand that spinach is 91% water. It is my hunch that the same volume of water would cool much faster. If 9% of spinach is attributable to other compounds, my first guess might be that water would cool only 9% faster than spinach, but I think spinach would cool at a much slower rate than that. If true, would the complex shape of rolled spinach be the primary factor in its heat retention ability when compared with water?
You may need to test your hunch to see if you are right. Water has a very high specific heat, so it is a very good reservoir of heat. A timer and a thermometer might say that it cools off just as slowly.
However, two things might make a "ball of water" cool off more quickly:
Evaporation. If the whole surface of the ball is exposed to the air (and especially if the water is well above room temperature), then evaporation can cool the surface quickly. The spinach likely inhibits a lot of this by being between the air and most of the water.
Convection. If the water was just in a pan or a ball, then currents could move warmer interior water to the surface where it could cool. With the spinach in the way, these currents aren't possible. Any hot water in the center has to cool by conduction. This is much slower than if convection were present.
The shape is probably important, but the complexity of the shape should not be. Just that it inhibits those two processes.