Why is water a bad conductor of heat ?
When you heat water at its bottom, the water expands, becomes less dense and is the reason why water is bad conductor of heat.
Water and air expand and contract more than metals. Expanding water molecules when heated separates the molecules and heating of water takes longer time.
However, compared to other substances like wood, glass etc water is a better conductor of heat. In water the heat is conducted to other portions via convection.