No-one can ever be sure. Book a Cabin that's midships and fairly low, take some wristbands and enjoy your holiday!!!!
No doubt you are worried about the sea conditions whilst crossing the Bay of Biscay. From my own experiences you just have to take pot luck. I have traversed the Bay in both directions in December and it was like a mill pond. However I have also crossed it in Summer and we had Force 8 gales. Hope this information is of use to you.
I think 'variable' is the most accurate estimate. At least it will be warm in the Canaries. Looking at the grey sky here I'm tempted to book something now!
Having crossed the Bay of Biscay on the Pride of Bilboa in a Force 10 storm I would say that there should be nothing to worry about. My wife is not the best traveller but did not suffer sickness - we were aware of the movement but the movement was so slow that although it was a bit difficult to walk about in the worst of it we were not aware of anyone being ill,
A cruise ship is much larger than the ferry and should therefore be much better able to cope if you are unfortunate to have really bad weather.