It's a fairly short crossing molly, but nobody can get at their cars once they are secured downstairs. Ferries are fine these days, they have stablisers once they leave the harbour, but people do still get seasick if the weather is rough and you can't always predict that, the Channel can be choppy. The best you can do is sit down or lie down and try to sleep - and if you can (and if you are worried about seasickness) sit where you can see the horizon, so your brain adjusts - if you are below deck it's more unpleasant. However - thousands of people use our ferries every day and thankfully they are now as safe as they can be, so plan to make the best of the journey, and have a good time!