Calais is just a stones throw away from Dover. 75-90 minutes on a P and O ferry will take you from Dover to Calais. Travelling by ferry can be advantageous to many and a preferred option for lots of holiday makers. Most people prefer to travel by ferry because it gives them the option of bringing their car, but there are also many options for a foot passenger from Dover to Calais.
Going on a ferry is much more relaxed and there is much more you can do than if you were travelling on a plane or a train. Enjoy the sea air and views, go for a bit of retail therapy and shop till you drop or go for a bite to eat at their cafes and restaurants.
If you are travelling as a foot passenger it is important to know that check in closes 45 minutes before departure. It is also worth noting that not all the crossings allow foot passengers. When travelling from Dover to Calais foot passengers can only go on ferries departing between 8.10am and 7.35pm. When travelling from Calais to Dover foot passengers can only travel on ferries between 6.30am and 10pm.
By booking two weeks in advance you would be looking at about £25-£30 each way when travelling as a foot passenger. Their website is quite easy to use (poferries.com) and contains a lot of information on bookings and advice on places to visit and accommodation and you can organise breakdown cover and travel insurance through the website.