I took the ferry from Britain to the Netherlands, and I'm v glad that I did. It's really relaxed, much less stressful than the airport, plus you can wander around the ship instead of being confined.
Ferry facilities are pretty tacky (the duty-free shop, the "teenager zone" which is a couple of Pacman machines, the roulette table), and I'm surprised they didn't provide internet access in the cabins (surely that's something they can beat the airlines at?), but it's still quite pleasant. The food was surprisingly good too: I had a really nice beef stir-fry, cooked to order.
The ferry ticket cost me about £100 return, including the trains at each end and an overnight cabin, so that's a pretty good deal. I even got free breakfast in the morning, which I hadn't realised was included.