If it's the sea that bothers you then handygran's idea of a river cruise is a good one. They are widely available in Europe - not just the Rhine, but the Danube, Elbe, various Russian rivers, the Gote Canal in Sweden, etc.
Another good minicruise is Portsmouth to Bilbao and back with P&O or Plymouth Santander with Britanny Ferries. The ships are large enough to to feel much motion.
Also some of the large cruise lines (eg Cunard) occasionally have mini-cruises out of Southampton to a French port and back..