Thanks for your question, Tuggy... The answer is 'no'... you don't have to keep reapplying the ceramic. I have enjoyed a 'hydrophobic' windshield (and rest of the boat, for that matter!) for almost 6 months now, and it's working as well as the day Kevin at Premier Custom Ceramic Coating put it on!
There is a 'serum' that should be applied about twice a year, and I have yet to do that due to weather. It's super easy though... just spray on and wipe off.
When I was using RainX, I was having to reapply pretty much every outing - depending on how much salt spray I got. It might hold up better in non-salt water, but it just wasn't a good fit for me. Ceramic definitely is!