it all depends on what level of runner you are, and what type of sand makes up the beach you plan to be running on.
Start on the hard stuff first, just on the edge of the water. I prefer bare foot, but beware of the friction of the sand. You wont notice it until its too late when blisters develop under your toes. So start with short runs, of nothing more than about 10kms until you are comfortable that your feet are used to the extra friction. Obviously make sure it is a clean beach, with no or very few shells, rocks, blue bottles, glass, or driftwood that could cut your feet.
It also can place a lot of strain on your ankles and muscles below the knee, so be warned... you may be stiff in new places the day after. Another reason to start slow, and short, and build up as you feel comfortable.
Once you are used to it, it is one of the most rewarding places to run! Good luck, and enjoy!
I've tried both, running with and with out shoes. Recently I ran a very short distance, 2Ks perhaps, up and down a beach, in shoes. The sand varied from soft to hard. My Achilles is now just starting to feel better, a little tight in the morning but no pain any more when I walk, run, or do jumping type movements. When I ran bare foot, I didn't have this problem.
We've recently arrived home to Australia, (from the great state of TX) and I love the beach. I'm hoping to run it again soon, but this time I'm going bare foot, and will stay where the sand is firm.... down by the waters edge. I haven't run in a month, and I am missing it. I have a friend visiting from the USA next week.... and we hope to get in all kinds of runs. Lots of different terrain here, where I live.
I have been running barefoot in Miami for over 3 years now. It has been a blessing and the only way I could continue to run as road running was killing my knees and aggravating the bone spurs on my heel area. I run in the slightly deeper sand but it is true that you have to start slow and with short distances and not when the sand is HOT! After a few weeks you will build up a tolerance for the sand but even then in South FL if the sand is burning you can end up with blisters and should wear shoes at that time of day. I know a lot of people are not able to run barefoot but if you can't then you will probably have to stay up on the harder sand area or the sand will end up in your shoe which is not very comfortable. Also if you are running down by the water, there is usually a slant and you should run the same distance both ways so that you don't have uneven use of your muscles. I don't think that sand running is the reason for the plantar foot problem, you can get that from overuse or maybe going too hard at the beginning of the program whether you are wearing shoes, running or walking. Hope that helps.