I've done it dozens of times in several different aircraft.
Being based at Plymouth I just take the direct route and it is about 30 mins to Guernsey, 40 to Jersey in my Mooney.
Decisions as to how long an overwater leg to take depend upon characteristics of pilot and aircraft and also prevailing weather.
For a newbie PPL it really feels like a big deal and gives a tremendous sense of achievement at having made it over the water without having the hassle of ferries etc...
Good luck with your flight.
The aircraft does not know when it is over water.
SB