Sun Screen: Riemann P20
I saw this advertised in a private travel magazine some years ago and thought, yeah, yeah, like you do. However the woman explorer clearly wasn't a well enough known name to be using her name as an endorsement, so I tried it.
You "paint" it on and it dries and forms a patina on your skin. Once dry, you can go into full sunlight, swim, walk, loll about, whatever. It only needs one application a day and I haven't burnt yet, despite using it on nudist beaches in the Cote d'Azure, Fuertaventura and Cyprus. So yes it works even on the sensative bits that are normally (mainly?) left covered.
It means there's no putting slap on as soon as one hits the beach!
There's only one downside, and that is that it takes 90 minutes to dry completely, 45 minutes for most purposes, and whilst drying will stain anything it comes into contact with, except skin. At the end of the day, the thorough use of a shower mit is a must!
Available at Superdrug and in duty free at Gatwick, amongst other places.