Marinas have secured parking area for cars. The price to park your car for a week will depend on the specific marina, but it will be around €45/week. It is not necessary to reserve a parking lot as there are plenty of spaces. The marina personnel oversee the car parking as well as the boats.
There are often cheaper parking options a short distance away from the marinas, although these are less secure. Check out the destination guides, near the base marina you are sailing from, for further details.