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To go to France, you have to show your passport. I don't know how exactly that works if you swim across... but in any case, if you can (physically) swim across, then rowing or sailing across must be (relatively) easy.
Assuming that such a sailor (a British citizen) were not doing anything (else) illegal, would he be permitted to travel to France in his private boat? Can he land on any old beach, or does he have to go to a port to identify himself?
Is it legal to cross the channel in a private rowing boat or sailing boat?
Yes.
Can he land on any old beach, or does he have to go to a port to identify himself?
It depends.
Who is on board, where did they come from, what have they got in their pockets, etc.
This may be helpful
Notice 8: sailing your pleasure craft to and from the UK
When arriving direct from a country outside the EU (the Channel Islands are regarded as outside the EU for this purpose), you must phone the National Yacht line on Telephone: 0845 723 1110. You will need to inform the Yachtline if any of the following apply:
etc etc
Since France is in the EU, whether you need to phone the NYL to report your departure from the UK depends on a complicated bunch of conditions you can read in the link above.
Maybe the French have similar rules?
The RYA say
Some countries specify ports of entry (ports where one may lawfully enter a country), which should be used by a vessel arriving from abroad. It is often a requirement that you proceed directly to such a port of entry on entering territorial waters. A vessel arriving in a country from outside its customs territory should fly the Q flag until it has been given clearance from the authorities. Even once clearance has been given, some countries may ask to inspect the vessel’s papers periodically, for example at each port of call.
and so on.
The UK is not a member of the Schengen Convention with respect to open borders. Therefore anyone travelling from the UK to France is required to report to customs and immegration on leaving the UK and arriving in France. How they travel between the two nations is entirely up to them.
