Depends on the time of year, summer being the most kind and the other months are, really, pot luck. It is not the friendliest of oceans but it's certainly not the worst. The biggest misconception is that you will have fine weather and this so often proves wrong with mists, fog and clouds conspiring to often make this a cruise to stay inside. The further south your route takes you then that can make a huge difference to the weather but the normal direct transatlantic is often a disappointment weather wise. The most stable are the "Atlantic-ocean" liners, ie. QE2 and QM2 they have been built for the job. Neil Down.
Hi we went from UK to NY on QM2 at the end of October last year and the crossing was quite good but we did have a couple of rough days and there were quite alot of people confined to their cabins. Even though the QM2 is a very large ship the Atlantic can be very choppy and there can be some bigs swells. If you do get sea sick take pills or there is a doctor on board and he will give you an injection which works and coats about £45 but worth it.
Also we did run into a fog bank just as we were coming into NY which slowed us down bit but still go to NY on time. Hope this helps.