Well, I must say that the Med is usually calm in summer, but the last year when I took the ferry home from Palma and stopping in Ibiza in September, the sea was considerably rough, and as it was the relatively small fast-ferry, it was constantly rocking up and down, enough to hardly walk along the corridors without leaning on the walls or the seats. Sometimes when the boat came down it felt like it was a rollercoaster,
It was quite funny because, the stewardesses were constantly handing out sickness bags to the people, and they told many people to lie down the floor at the back side of the boat to recover.
A few dutch guys who got up in Ibiza (their faces revealed they just finished the party before getting on the boat) intended to go on drinking alcohol on the boat, and some of them needed to lie down at the back side.
However it's not likely to happen in summer at all.