I tried to put some numbers on this when I was considering over water flights in my microlight. When your single engine is a 2-stroke with 1000 hours on it and your speed over the water is only 40kn it doesn't take much of a water crossing to get you anxious!
There's a lot of guessing here, but the orders of magnitude should be about right. My 2-stroke had had 2 failures in 1000 hours, and speaking to other microlight pilots with a lot of 2-stroke time an engine failure about every 500 hours seemed a reasonable guess. If half of ditchings are survivable (probably pessimistic) then every hour out of glide distance of land has a 1:1000 chance of ending badly. The risk of a fatal crash in GA (and microlights are similar) is about 1:50000/hour flown, so flying over water out of glide distance in a 2-stroke microlight is about 50 times more dangerous than flying over land. So for me a cross channel trip at 5000ft with 10-15 minutes out of glide distance from land was as risky as 10 hours of normal flying, (or 2-3 months of my 3 hour round trip commute to work by car).
A more reliable engine and a faster plane with a better glide makes water crossings safer, but it should still be possible to frame the question in terms of "how many hours flying is this crossing as risky as" and get an idea as to whether its a risk you want to take.
Last edited by hollo; 20th Mar 2016 at 22:59.
Reason: typo