I googled it and it seems like it is legal ANYWHERE up to the high water mark. As stated, sometimes asking you get a personal opinion and not the law.
The public beach does NOT allow metal detecting per their rules:
http://dauphinisland.org/public-beach/
However, most laws are ONLY for land and not the water to the high water mark. I don't believe ANY state park owns the ocean.
Welcome aboard. I hope this helps.
It is like if you anchored a boat off shore and detected, what could they do? You are NOT in the park. Maybe try googling RIPARIAN rights in Alabama to get a better answer.
Example:
A few years ago I was at a lake with a public boat launch in Michigan. We launched my boat and went over to a private marina. They had a roped off swim area. I got suited up and went in the roped off area to detect. After a while I felt a tug on my dive float. I came up to a woman on a float. She told me I couldn't be in that area. I told her she was wrong. a guy on the dock yelled, have you ever heard of riparian rights? I yelled back sure have and you don't have any. I told her I wasn't leaving and if she didn't like it to call the police. She also said I couldn't anchor my boat where it was. I told her she was wrong there also.
I went back under and a while later I heard a bam bam bam. I came up to a police officer on the dock. I walked closer and he told me I was trespassing. I told him no I wasn't. He said well now that you are standing up you are. I said no. This is a public lake with public access. No home ower owns the water up to the high water mark. He said he was reading the law and he believed I was right. I told him politely I knew I was and told him my background. We had a GREAT talk and he left to tell the folks I was right they were wrong.
So the campground can make the land illegal to detect and state NO METAL DETECTING. BUT the ocean is NOT theirs neither is the sand under it. You are not in the campground as soon as you touch the land at the high water mark. Plain and simple. Nobody owns the ocean as far as I know. I would be shocked if a campground had any rights at all.