The waters off the west coast tend to be cooler than those on the east coast (especially in the Northern Hemisphere summer). Winds blowing over water will reach a temperature close to the water temp. The air near the surface of the water is close to saturated. The dew point temperature refers to the temperature you’d have to lower the air temperature to to reach saturation. Example: If the temperature is 80 and the dewpoint temperature is 75, you are dealing with some extreme humidity. But, if the air temp is 80 and the dewpoint is 60, not so humid. The last example is what the west coast experiences. Air over the ocean is saturated at about 60 degrees (the temperature of the water). As that air moves over the hotter land (lets say the land is 80), the dewpoint is low. Meaning low humidity.
On the east coast and the Gulf areas, the water temp is much higher (using my example, lets go with 75 degrees). The air over this water is saturated at 75 degrees, and even if the land temp is the same as it is for my west coast example (80), the dewpoint over land is much higher.
Does that make sense?