why does a thin layer of water evaporate quicker than a thicker one of the same surface area ?
If both bodies of water had the same surface area then the thinner one would have less water. Less water would take less time to evaporate.
Water also has a high specific heat, meaning it can absorb a lot of energy before it heats up. In the thicker layer of water, the water at the center is insulated from the heat source, and needs all the water closer to the sun to evaporate first before it has a chance to. This is the same reason why blankets work: energy has to pass through the insulating layers, and the more insulation you have, the less energy will pass through.
Think of the thicker one as many thin layers of water. If the thicker layer is 5x as thick, it’ll take 5x as long to evaporate, all else equal.
Are the two bodies of water the same volume? The same surface area but deeper is more volume so it’s gonna take longer all else equal.
Evaporation only occurs at the surface, so increasing the surface area to volume ratio will accelerate the process.