Generally with photography all that is needed is a bit of commonsense, with the advent of online albums and effectively world publication please consider other people.
Although in the UK it is legal to take a photo of someone in a public place, think about how you would feel if you suddenly got a call from your boss to say they had just stumbled across a photo of you naked in the background of another persons photo.
Follow the following guide and everyone should be ok.
THink about whether moving to take the shot from a different angle would ensure no one outside your party is in the shot.
People that are in your shot, if they are not facing the camera, ie. have their back to you, would they be recognisable, if they are think again, or ask them if they don't mind, they may well accept, after all they will know how it is nicve to take a photo for memory lane.
Constantly taking photos or using binoculars may get you some nasty reactions and/or being banned from a beach.
Remember in this day and age, if children are in the shot, may be some children from the another family who are all building sand castles with your children then ask the other children's parents before even reaching for the camera, they may be just as hesitant to ask you if they could take a family snap.
Videoing and camcorders are generally regarded as more intrusive than still cameras, so don't video unless you are sure no one else is in the shot.
Finally remember people are not mind readers, and so being open and telling people and chatting to them about what you are doing rather than a furtively taken photo is a better option, and it'll help you make new friends too.
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