There are quite a few microlight flexwings and 3 axis which fly from Folkestone to cap Gris Nez ( 22 miles) which is the shortest distance over water to France. Most travel speed is 65-120 mph. Most will travel over the water at 80mph with life jackets etc. Grant it the channel is busy and we probably fly at 4500’ possible glide back to land. North Sea less busy.
in June 2019 a group on the fly-UK did the 28 mile hop from Scotland to Northern Ireland I cannot remember the exact route taken. Again these were in microlights.all had immersion suits just on I believe.