The best thing I can say about preparation is "start early". It's possible to just take the horse to the beach and teach him to cope with it as you go on your ride, but you have to be skilled enough that you can handle almost anything. That's a big part of being successful at riding outside. Knowing that you have the skills and the composure to cope with anything that happens. The beach can be an overwhelming environment for a horse to deal with; their senses are being flooded with a thousand stimuli.
When I was learning how to start horses for trail riding and I wasn't yet a good enough hand to just get on and go I would take them on foot for walks. It's not exactly the same for their education as riding them out, but it was a good intermediate step between the arena and the trail. Once they were nice and quiet, responsive consistently no matter where I took them it was a piece of cake to get on and ride.
As for riding on the beach, I like to make huge circles as I go along and look at my tracks in the sand. The more symmetrical the circle is, the more I know that my horse is tuned into and listening to me. I'd alternate between riding circles and straight lines, do some galloping, all kinds of different stuff.
Lastly, I found out that it was a good idea to use caution when introducing the horse to the water. Surprising things can happen when that tide rushes up to their feet for the first time and they've never experienced it before!