Hi Mary,
the beach that I have operated in and out of was at Pebble Island in the Falklands. In the winter if the grass airfield became to soft to use then the beach could be used. We would use to tide tables for planning and avoid it between an hour before and after high tide to allow enough width. The area just below the high tide mark gave the best compromise of firmness and dryness althought the aircraft was washed at the end of a day that included beach ops. Salt and sand are about the worst mix around for anything mechanical.
We used a cb radio (2m band) for talking to the folks on the beach to get info on surface conditions, they would park the land rover to give us a threshold mark of sorts. After a strong northerly gale kelp build up on the beach could be problematic at times.
As for cargo, it was a mix of whatever was needed, in my time I have flown anything from royalty to pigs although the trickiest was probably a 350 kg grass mower.