Regarding the concern about engines over open water, I read long ago that many engine problems arise when something is changed so I have always adjusted settings and made whatever checks I thought appropriate before coasting out and then tried to fly a steady course to my landfall. So far so good.
'Automatic rough' over water is, of course, nonsense but I have found that the air tends to be relatively smooth and non-turbulent on sea crossings so perhaps the engine note may be more apparent in the relative calm, giving rise to the idea but mostly I think it's nervousness from being out of one's comfort zone.
The channel short crossing to Cap Gris Nez, which I imagine would be your initiation to the business, is relatively short and is probably quite busy with sea traffic. There are longer sea crossings available within the UK much less frequented by shipping below and they can feel quite lonely.