In the west coast of the United States, the coastal areas usually, but not always, will experience warmer temperatures than the inland areas in a fair weather day to mainly to the following reasons.
For the cool season: The cool ocean temperatures along and just off the coast usually range from the mid 50s in the north and lower to middle 60s day and night. The airmass along the immediate coast at night will cool to at least nearly the same temperature of the water surface temperature. However, if you move farther away the strong influences of the ocean, you will experience colder low temperatures as heat will escape due to radiational cooling during those long winter nights. This fact is important because in most cases, by sunrise, the water and the air just above the water will likely be at least slightly warmer than the immediate surrounding enviroment. With slightly warmer temperature at the low level, you will start the morning with a slightly unstable air mass along the coast. This will keep the air mixed and will likely prevent the coastal inversion from forming. In other words, the mixing will prevent the low clouds and coastal fog from forming. After sunrise, the surface heating from the sun will keep the air mixed along the coast and will likely keep the skies mainly fair for most of the day.
However, the inland areas will have little or no influence of the ocean temperature so it will cool to a much lower temperature to start the day. This cooler low temperature can result in the formation of fog and low clouds during the early morning hours. If this occurs, it will take time to burn off this fog and low clouds. This would limit the amount of daytime sunshine. With less sunshine, the inland areas will likely not warm up as much as the coastal areas.
For the warm season: The cold waters of the eastern Pacific Ocean and the strong upper high pressure that dominate the weather along the west coast. This will usually result in the development of a layer of marine air as the cooler ocean temperature will produce a very stable layer above the ocean surface.
Unless there is an offshore flow, this marine air layer is usually between 1000 to 2500 thick in height (above sea level). The top boundary of this marine layer exist a temperature inversion. Since the water temperature during the warm season will remain in the upper 50s to 60s, the marine layer is very stable and very moist. As a result, the top of this marine layer you will find a layer of thick stratus clouds. During most of the warm season, this layer will move slightly inland with the sea breeze and will stay for most of the day. Sometime it will "burn off" in the early afternoon and sometime it will stay cool and overcast all day. If it stay overcast, the coastal high temperatures will most likely remain in the 60s. If they get some afternoon sunshine, it will be in the lower to mid 70s. If it is an offshore flow, it will be sunny and temperature can climb into 80s or 90s.
In the inland areas, the you will just bake under the summer sun. So the highs will range from the 80s to around the century mark...unless you are lucky to get a sea breeze moving far inland during the day.