Depends on the SN luckylady. DS1 doesn't understand about queing or sitting still so flying is out. No amount of preparation is going to help with the way he behaves in strange places. He udnerstands about sitting in the car but he freaks if the car stops. So sitting still waiting for a plane to take off just isn't going to happen. If you try and force him to sleep you get a full blown screaming attack- and that's not tolerated in toddlers let alone 7 year olds.
The last ferry trip we took (to Belfast) was exactly that- the last, and that was before ds3 was born. The trip over we took took a slow ferry and had a cabin. DS2 stayed in the cabin with one of us and the other person had to walk round and round and round the ferry with ds1. It was OK, although tedious. The problem was it was a 16 hour drive to the ferry and from the ferry at the other end and staying in a hotel was a disaster (ds1 wouldn't sleep, and waqs screaming and clawing at the door to get out- after an 8 hour drive that day dh had to drive round north wales for 2 hours to calm him down- we almost turned around and went home at that point).
On the way back we took a fast ferry so we could do the 16 hour car trip and not have to stay anywhere and it was a disaster. No cabin at all, and ds1 spent the whole time trying to get into the gift shop through the exit, and look under toilet doors. He didn't seem to even notice that the ferry was moving. DH went green (as did ds2 luckily) and I stumbled round 6 months pregnant trying to stop ds1 sniffing and pinching people. With ds3 as well it would be completely impossible. We drove off the ferry and dh rang his mum and said there was no way we could come again. until some sort of miracle happened.
The Stranraer big fast ferries are better, but no way we can do the trip in one day and we can't stay anywhere like a hotel.
Just under 3 years later he's even less manageable in that sort of situation I think.
Mind you we don't take ds1 on UK holidays either.