I also took the " Pride of Bilbao " to Bilbao, drove to Biarritz for 10 days' surfing with my kids, and then back to Portsmouth. Even in August the sea was rough.
My recommended OTC ( over the counter ) drug for motion sickness is Cinnarizine ( " Stugeron " ). It's an antihistamine, and should be taken 30 mg two hours before travel, then 15 mg every eight hours during the journey if necessary ( half that dosage for child 5 - 12 years ; avoid during pregnancy ). The only one in our group who didn't take it was seasick !
http://www.netdoctor.co.uk/medicines/100002475.html