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If you are really worried try a fjords cruise. If you don't want to cross the North Sea you could fly to Norway. We have crossed the North Sea four times and had calm seas every time (we are probably due a bad crossing!!!). As Mr/Ms Taylor says the remedies are available. I went to the doctor for tablets, but then discovered Stugeron are the same thing as he prescribed and its cheaper to buy them than pay for the prescription. You need to take the tablet before you feel sick for the best results, so if the weather forecast is doubtful I take a tablet. I hope you enjoy which ever cruise you choose.
The answer probably lies in the time of year you plan to go and whereabouts you'll be cruising. Several years ago we cruised from Southampton down to the Canaries for a Christmas cruise. We hit some bad weather in the Bay of Biscay (believe this is usual for this area) but other than that on most ships if the weather is anything other than a storm of some description, it's normally like being on land!
There's a bit more to it than that. Some people are more prone to sea sickness than others (obviously) and some ships are more stable in rough seas than others (again obvious). So it's not possible to answer the question. But of course cruise ships are well used to this problem, and between them and you you can do a number of things. First of all, book a cabin in the middle of the ship and low down. Secondly, try travel sickness remedies or preventatives. Bangles used to be all the rage, and ginger can be useful if you don't like taking drugs. Otherwise there is a range of antihistamine type tablets available at your local chemist. Some of these can have sedative effects. All ships will have stocks and also offer injections which are very effective, but will have to be paid for.