Although at times it has been possible to boat down the streets of Lewes for something a little smore fun there are boat trips at Eastbourne and Brighton Marina. Operators come and go so check with the tourist office before you set off
Since the end of duty free, ferries have become increasingly reluctant to take foot passengers. From Dover, only the P and O service to Calais takes foot passengers
From Newhaven, DFDS normally takes foot passengers, but they have been known to to limit or stop foot passengers in peak seasons. Check well before you plan to go. Tickets may be available at the ticket office in Newhaven even if they are not available online. You could also try their phone booking line at 0800 917 1201
When you can book the return fare is usually £30 in 2018
The problem is that there is no planned link between the ferry and the trains on either side, and the railway station at Dieppe is some way from the ferry terminal, although there are shuttle buses.
It will probably take 8 hours to Paris. Check Loco 2 for the French train times.
The attraction of the route is that it is more romantic than Eurostar and you can do the trip to Paris for less than £100 return even if you only decide to go on the day.
From Dover, all three ferry companies take bikes to Calais. There is great cycling south and south west of Calais.
DFDS will also take your bike to Dunkirk for the same price. The terminal at Dunkirk is a long way from the town in the middle of a range of docks and industrial estates, but a map will show you a way into the town avoiding the busiest routes. One cyclist reports that even the main routes are quiet if you wait until after all the ferry traffic has left. From the town it is a very short ride to Belgium, with its wonderful cycling network (almost as good as the Netherlands and just as flat, but with better scenery and beer.)
On the Newhaven/Dieppe run in principle bikes are allowed on all ferries, but when I checked one August bookings the site said that the boat was full for cycles on some sailings, even when the ferry was taking bookings for cars, which take up much more space. This suggests that you should always book in advance for peak periods. Tickets may be available at the Newhaven terminal even if they are not available online. You may also want to try their phone booking line at 0800 917 1201
Normally you must make each online bike booking one at a time. This means that if there are four of you then you will have to book four times.
Off peak, particularly on Dover routes, it can nearly as cheap (and sometimes cheaper) to drive over with your bike, park your car and set off on the bike- not so ecological, but environmentally better than driving all the way to Cornwall, or flying.
Or you could pick up a Flixbus coach at London or Dover to your destination
Bicycles and Eurotunnel (another way to cross the channel)
Twice a day you can take your bike by Eurotunnel
The times and the pick up points are not particularly convenient. You must book by phone and there are only 6 places.