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You will be fine, just take some tablets just incase we have done biscay in June, weather excellent but much different in November. You should be ok in July/August. Cruising is wonderful not to be missed. We are going next Sat on Sea Princess, can't wait
We have crossed the Bay several times and most times it was like a mill pond. On the occasions when it was a litle rough I took a travel pill, went to sleep for a couple of hours and then i was fine. My husband does not suffer at all.
My late mother went on many cruises in the 1960's and was, without fail, seasick on every single one but she still went on them because she new that by the third day she would be fine and have a great holiday. I took her on Oriana a few years ago 30+ years after her last cruise and, much to her surprise, she was fine, despite there being a bit of a swell. Yes I know being seasick isn't nice (I take after my mother!) but you are letting the thought of one particular day cloud your decision to take her on a great holiday. I would recommend a cabin on a lower deck about two thirds of the way back (but not a deck immediately above the engines) to minimise the effects of any rocking and pitching you may encounter. I also find getting out on deck in the fresh air makes me feel a lot better than staying in the cabin. Book Now!
As the previous answers indicate nobody can guarentee the weather conditions, although the odds are that you would be ok. However you do have alternatives if you like to cruise the Med. Take the overnight train to Barcelona, where a lot of cruises leave from or take the day train to Nice in S.France, stay there for a night and then get the train to Savona in Italy(abour 90 mins), where Costa cruises depart from. We do the latter and regard it as part of our holiday and the great advantage is that you are at the interesting end of the Med in my view eg Italy, Greece, Turkey, Malta et.
Take the tablets they give you on board the ship. I was ill but after one tablet and half an hour on my bed i was like a different person.