A friend of mine has just booked with Ryanair, UK (one of the London airports) to Venice (I think, though possibly Ancona) for 1p each way in May. So even with the price of the ferry crossing it can be a very affordable way of travelling if you don't mind the additional time. Having said that, the view from the ferry leaving/entering Venice also makes the trip very worthwhile.
Just wanted to share ('cos I'm a happy bunny at the moment)... although the easyJet flights have gone up dramatically, in the last 1/2 an hour I've just booked 3 seats Corfu to Gatwick on 14th September, returning 2 weeks later, for a total cost of 258 euros (just over £190). We travelled over in June last year and it cost nearly that much per head - and the flight times weren't so good!