Answer
Water moderates the temperature of nearby landmasses, acting as a heat reservoir.
Work Step by Step
When a substance cools relatively slowly, it has a relatively high specific heat capacity. Water has a relatively high specific heat capacity compared to other common materials. Another way to phrase this is that water has a lot of "thermal inertia".
For places near large bodies of water, the surrounding water moderates the temperature, acting as a heat reservoir. Water is slow to warm up and slow to cool down. In other words, water gives off and absorbs large amounts of energy for small drops and rises in its temperature.
In the winter, when the air is colder than the water, it is warmed by the water, resulting in a milder winter for the region. In the summer, the reverse happens and the water cools off the air, resulting in a milder summer for the region.
