Unless you go a very long way round (e.g. on a massively-expensive world cruise, taking several months, via India and China) it's impossible to get from Southampton to any part of the Caribbean without crossing the Atlantic Ocean (which will take several days in each direction).
Any route to the Canary Islands is going to pass along Spain's Atlantic Coast (which is why most cruises call into ports on the western side of Spain). Unless a cruise (unusually) includes ports in France or northern Spain (such as Bilbao or Santander) the ship won't have to go into the Bay of Biscay, per se, but it will follow a line along roughly where the Bay of Biscay meets the Atlantic Ocean.