My family goes to the beach every year in October. We go to Assateague island.They allow riding on the beach from mid October thru Memorial day(?). They have a 50 horse limit so we try to get there early in the am.(it's a 4 hour drive for us) We stable our horse's at a nearby race track for the night and usually go for another ride before heading home the next day. This is a state park as well, perhaps in your area there is beach property that is parkland and they may allow you to ride on it.
Happy trails! Denise and gang