Mal de mer is the queasy feeling many people get when they're on a boat or ship. You might feel fine on a large cruise ship, but suffer from mal de mer when you're on a small ferry.
Mal de mer is another way to say "seasickness." It's a particular kind of motion sickness that afflicts people traveling by water, and it causes dizziness and nausea (a word that actually means "seasickness" in Greek). Mal de mer is brought on by the rocking motion of a boat on the waves — although not everyone is affected by it. In French, mal means "bad" or "sick" and mer means "sea."