Why do we enjoy lower humidities than other parts of the U.S. when we're so close to the ocean?
It's because of the sea surface temperatures off our coast, in contrast with those of the Gulf of Mexico or Atlantic, which supplies moisture to the air east of the Rockies. Sea surface temperatures off the Oregon and Washington coasts typically are in the upper 50's to low 60's this time of the year (Puget Sound is much cooler, the low 50's!). Sea surface temperatures of the Gulf of Mexico and the Gulf Stream off the central and south Atlantic coast are in the 80's. The warmer the air, the more moisture it's capable of holding, and the areas that are downwind of air moving over the Gulf of Mexico or Atlantic see much warmer and much more moisture.
Incidentally, I should mention that relative humidity isn't the real measure of interest. It simply relates the amount of moisture the air is holding compared to what it can hold. Let's say Seattle has 70% relative humidity and Miami has 70% relative humidity. The air in Miami may actually hold double, or more, water vapor compared to Seattle...because the temperature is much warmer there. What meteorologists really look at as an absolute measure is something called the dew point.