I must admit that one of my favorite dreams was riding a horse at a full gallop on a beach. Since I do know how to ride and have ridden horses since I was very young, I know that it is a dangerous dream.
You see the sand on the beach can hold dangers for any animal running. You know how hard it is to walk on the beach when you sink too deep in the sand. The horse would end up injured if ridden in the wrong part of the beach at a gait faster than a walk. The high beach is too deep and the break water area can have soft spots where a horse's leg could sink during a gallop and cause the horse to break a leg.
Those dangers does not seem to deter those people who do gallop horses on the beach during times when I know the sand is treacherous. One day I saw some english horse tack half buried in the sand. I found out later that a horse got injured and the tack had to be cut off of the horse before he could be lead off the beach. I have no idea if the horse survived the injury.
If you like riding on the beach and would like to keep enjoying that activity, then walk your horse and go no faster.