they can get quite crammed, but you may need to arrive early in the day, like between 12 and 1 when all the leavers have gone but before the arrivers have arrived. North french coast surprisngly uncrammed, imho, esp fab places like Deauville.
Cherbourg boast that they have never turned a visiting yacht away. St Peterport gets pretty full, and you might not get into Victoria Marina, but there is usually room to squeeze in somewhere? The pontoons in the harbour mean you must dinghy or watertaxi ashore - all of 50 metres...
Although they crowded during the school hols, you will get in St peterport, St Helier and Alderney. It's not long ago that I my kids on board during school holiday times and we always got in somewhere. If bad weather turns up St Peterport can be a real squeeze.