I've got a coworker that does this shit on old battlefields. Dunkirk is his preferred hunting ground, since the allied forces dumped soooo much into the sea when retreating.
From memory, he has found Vickers guns, Lewis guns, a metric fuckton of SMLEs, quite a bit of Mausers, buckles, medals, a lot of bandoleers, the occasional Webley, a Boys antitank rifle and so on.
He has also found minie balls, black powder cannons, 1700's & 1800's rings, and so on.
Nearly all of his finds were monetized. Most legally, but the Vickers for instance was rebuild before it was sold and is actually capable of shooting (which is highly illegal, I guess). When shit ends up in clay ground or bogs, it's often preserved quite well.
You also find lots of modern rings and money, from what he tells me. Stupid people lose a lot of shit at the beach.
The French have started more heavily cracking down on these kind of things though. Stuff like shipwrecks are now off limits. Probably for the best, since the rust creates razor sharp cutting edges on those wrecks.
Last weekend it was the lowest tide in 25 years (at 4 in the morning), so preferred time for going hunting for stuff. He didn't find much interesting stuff. Just bandoleers and SMLEs.