Because of water's fluid molecular composition, it takes longer to heat and cool than land, which has a molecular structure that is more static and dense. Convective movements and the thermal capacities of bodies of water cause them to have a longer heat and cool time than land. Additionally the typically darker color of land gives it a higher propensity to trapping heat.
Because water is a liquid, its molecules are in greater motion than those of land, a solid. Like air, water is prone to convective movements and vertical mixing. Because water molecules are in constant motion, it takes longer for solar radiation to uniformly raise the temperature of a given body of water.
Large bodies of water like oceans have higher thermal capacities than land due to their immense density. As a result, it takes up to four times the amount of heat to raise the temperature of a given amount of water 1 C as it takes to raise the temperature of the same amount of land by an equivalent degree.
Color is also a factor. Because water is lighter than land, it reflects more solar radiation and does not heat as quickly.
Sunlight penetrates many meters into a body of water, whereas it only hits the upper, superficial portion of land. That is why it takes longer for water to cool than it does for land.