Ditchings might not be very common, but there are is usually at least one every year in the UK.
AFAIK there have only been 2 microlight fatalities in the Channel and one died when run over by his would-be rescue craft. The other was in mid-winter and the pilot had not donned his drysuit.
Microlight pilots are aware of the risks, but you only have to spend some time at Abbeville on the Blois show weekend to see how many clacukate that risk to be small. On the Dover/CGN crossing, under normal conditions, a microlight is vulnerable to engine failure for under 10 minutes.