Giving a single answer to this question is hard as this depends a lot of the individual.
Being on a boat there will always be movements. A big advantage of the area where we sail is that the islands do provide shelter. We will always sail on the calm side of Formentera, where the waves will be limited. Also, the weather conditions in summer are normally good.
Knowing yourself, you can probably foresee if you are sensitive to seasickness. For example, if you are likely to get sick in cars, the chance of getting nausea on board is higher. You can read further below about how to prevent seasickness.