Water is a good absorbed the energy from the sun. The land absorbs much of the sun's energy as well. However, water heats up much more slowly than land and so the air above the land will be warmer compared to the air over the ocean.
The answer is differential heating between water and land. Water, like the ocean, has a high specific heat, so it changes temperature slower. Land, like sandy beaches, has a low specific heat, so with the same amount of heat, it increases temperature more quickly compared to water.
