You pay a helluva price in fuel efficiency to cover the probability of an engine failure over water in one of these
Everyone flying over the Channel should carry a raft. Good 4-person ones start at about £1000-1200. Harry Mandelsson is one place to look, also I would try SEMS directly. RFD also sell their low-end ones off their website and I think you get more for your money with those than with something which has been imported and been through the whole retail distribution markup chain...
Then wear a life jacket while over water. They are nearly all made by Remploy, to very slightly different specs according to who they are for. Cost about £60.
Annual maintenance is about £120 for the raft and about £20 for a life jacket. Not legally required, I believe, but advisable.
And an EPIRB. £500 is a start for a 400MHz unit. Very advisable if flying over larger expanses of water.
There are so many ships about on the average day one could probably glide (from 5000ft) right next to something. I never worry about it.