Lots of ferries are available, though it also depends on where you want to sail to/from. The Dover-Calais route is the most accessible to London; you can also sail to Portsmouth from Caen, St Malo, Cherbourg, and Le Havre. Lastly, if you're looking to hop between Paris and London, you shouldn't forget Eurostar, which goes under the Channel in much less time and for some very reasonable fares.