Hej Urban, valkommen!
The first town which came in my mind is
Sperlonga, it's moreless as you described it, and a really breathtaking place!
Google provides pics
Also, many of the towns in
Le Cinque Terre adhere to your wishes, and also some other town in that zone, especially
Lerici. To see pictures of it,
Google keeps providing
Then there are many many others, Italy has a lot of sea and a lot of medieval towns, so I'd say at least 20 or so towns are perfect. Most of Calabria is a fertile ground to find what you're looking for, also
Many of the most precious towns, though, probably have a very strict policy about allowing filmmakers to film there, you'd have to ask permissions and such I suppose!