The most accurate explanation for why the temperature of land near water is different than inland is:
The air over the water affects the air and land nearby.
This statement captures the concept of maritime versus continental climates. Water has a higher specific heat capacity than land, meaning it can absorb and store more heat without undergoing as large of a temperature change. As a result, areas near large bodies of water tend to have milder temperatures with less fluctuation between day and night, especially compared to inland areas that can experience more extreme temperature changes.