Despite competition from the Chunnel, services aboard ferries and hydrofoils operate day and night in all seasons, with the exception of last-minute cancellations during fierce storms. Most ferries carry cars, trucks, and massive amounts of freight, but some hydrofoils take passengers only. The major routes include at least 12 trips a day between Dover or Folkestone and Calais or Boulogne. If you're bringing a car, it's important to make reservations, as space below decks is usually crowded.
Timetables can vary depending on weather conditions and many other factors.
The shortest and by far the most popular route is between Calais and Dover. If you plan to transport a rental car between England and France, check in advance with the rental company about license and insurance requirements and additional drop-off charges. And be aware that many car-rental companies, for insurance reasons, forbid transport of one of their vehicles over the water between England and France.