SST and air temperature differences
You're right. Water has a much higher specific heat capacity than air, so will change temperature much more slowly. This means that, 


the sea surface tends not to reach the same extremes as the air (hence the seasonal effect that you mentioned) 
the sea tends to lag behind the air in seasonal changes 
the air readily heats and cools with day and night, while the sea does this much less; so which one is warmer can vary on a diurnal as well as a seasonal basis. 


All this can then be greatly complicated by sea and air currents, especially vertical circulation in the ocean - but you have the right idea.