I want to second the long leash not being a good idea.
I used to live at the beach. I was blessed with my first greyhound being the best dog ever. He was off lead at the beach all the time. The problem was he saw a greyhound out there on a very long lead and before anything could be said....the both got wound up and the dog on the lead got tangled up. It didn't do any serious damage but it could have. It was an off leash area so it was going to happen at some point with some dog wanting to play with the greyhound on lead.
I am not here to debate the on/off lead issue but sharing an experience so your dog doesn't get its legs tangled up.