I guess it really depends on the sandals, some are totally fine and it also depends on the shorts, there are some really nice shorts as well. Most of our friends dress nicely so I don't think we should be worried. I've just seen some pictures of weddings that it honestly looks like some people were lounging on the beach all day and then they went and sat down for the wedding hahah.
I agree, I think of our friends and family and think we will be fine. But then again I have seen these pictures you refer too!
I will make sure in my pre-travel brochure that I am clear that no 'swim/lounge' attire is appropriate.
Most people are telling me about the pretty dresses they are buying, so fingers crossed!