Currents (rivers of water that run clockwise in the N. hemisphere)
Overall, the pacific is warmer than the Atlantic simply due to its size (more surface area to be heated by the sun). However, due to the Gulf Stream current (off the US eastern coast), more warm water is pulled into the Atlantic from the tropics than is replaced with cold water from the Canary current (off the S. Europe and N. Africa coasts). Off the West Coast, the California and Alaskan current bring cold water down from the north pacific current as well as some from the Alaskan current in the Gulf of Alaska.
These currents are the reason hurricanes can ride up the entire east coast of the US, but rarely make it up to Southern California. Sea temperatures off the SoCal coast usually range in the low 60's (F) while sea temperatures off the S. US Atlantic coast can be in the mid 80's or so.