Planning to go to Tenerife soon. Any etiquette issues with taking photos of topless women on the beach? Is it normal to do? Best to ask first?
Not without a person's permission. Doing so without it is one of the reasons why you might not find as many women prepared to be topless than say ten years ago.
It's not acceptable in France or Spain or anywhere I've been. Women don't tend to sunbathe topless or nude in the hope that some guy is going to come along and take photos, with or without asking.
There is an unwritten rule on many nudist beaches that you don't photograph anyone without their permission. Families or couples taking photos of each other might be tolerated in some places, but even then you are probably photographing people in the background unintentionally, so best avoided.
Taking photos of other people at a (nudist) beach without consent is probably the best way to test out how your camera can stand being trampled down in the sand or thrown into the sea. If the original question us really serious, i wonder from where the op gets such a strange impression of customs and conventions in Europe.