I travelled back from the canaries to Southampton and got thrown out of bed 3 times in one night !! I matbe need to mention that I was in the foremost cabin, and were given another cabin midships to aide our sleep. I must admit it was great fun tho.
If you find you are affected by sea-sickness do NOT hesitate to go straight to the ships doctor and an injection for about £30 will sort you out for the rest of the cruise. There is no need to suffer. Needless to say the outbound journey was like a mill pond !!
We came back across the Bay of Biscay on October 27th last year and it was the smoothest I have ever known it to be. I've also been through it on many occasions in the high summer and some crossings have been dreadful. So basically the answer is, it may be rough but then again it may not. Just enjoy your holiday and, as they say, take the rough with the smooth.
Have cruised from Southampton into the Atlantic/ Bay of Biscay many times. Statistically, in my experience, it will be slightly rough one in four times, rough one in eight times and very rough (stay in your cabin and they bring you food&drink) one in 16 times.
Just got through the bay last week got a good bouncing if your mid ships its better sailed through there 4 times first 3 no problem this last one made up for it but your only in it for 24 hours or so have a pill and a stiff drink and enjoy.
A few people cruising with us were particularly affected, but visited the doc for an injection and came to dinner later!
There is just no way of knowing! I have just reurned from a Canary Island cruise on P & O's Aurora. It was force 9 in the Bay of Biscay outbound but fine on the way back. I am not a brilliant traveller when it is rough, and recomment 1) going on a decent sized ship 2) having a cabin on the lowest deck possible in the middle of the ship (also cheaper!), and 3) Sturgeron travel pills are excellent.