Why does the top 16th to 1/4 inch of water trapped in my canvas boat tarp freeze at temps above freezing ?

I have a boat that is covered with a canvas tarp that has water trapped in it from a previous winter rain. That water freezes at temps just below 40 degrees fahrenheit actually around 37 to 40 degrees. There is water right next to this in a plastic bucket that does not freeze, and also standing water on the concrete etc... that will not freeze. The only things around that get frost or frozen is the water in the tarp, the top of my car, shingles on the roof and the gr. So why will the water in the canvas tarp freezes above 32 degrees but the water in the plastic wont. I don't understand this. thanks for any answers. The air is usually very still on these nights and the skies are cleara and crisp, this is at sea level I live right on the coast, (Santa Monica, CA ). Does the material that the air come into contact with help to lower the temp a bit just above the specific materials(canvas, gr, etc...)?