Getting permission from the landowner is absolutly the best policy. However i have done alot of research in this. Now every state would differ my research is for Iowa. I do alot af kayaking and camping and I looked alot into it before I ever even found an arrowhead. I wanted to know if I could camp on a random sandbar wherever I might be along a waterway when it got dark. As it would be hard to ask permission or even know where you'll be by nightfall.
Most if not all states should have some sort of department of natural resources like iowa. Heres the regulation page from the Iowa DNR website for canoes and kayaks:
http://www.iowadnr.gov/riverprograms/regulations.html
There are only a few meandered waterways in Iowa, most are non meandered which means the landowner owns the land adjacent to and underneath the water-including the bottom, sandbars, and banks.. Technacly this means they own the arrowheads too. But there is a provission that allows activities incidental to navigation such as, fishing, swimming, and wading.
Its not what we all want to hear but, taking an arrowhead without permission from the person who owns the land could fall under theft and tresspassing. Somewhere I read that the deffinition of tresspass in iowa includes going onto ones land with the intent to take, alter, or damage something without there permission.
From what my farm insurance agent told me that a landowner is not liable for people he lets hunt, fish, 4wheel, or whatever on his property. UNLESS he accepts any form of payment, including cookies or deer meat. If he gets stuff like that it is considered liable to the insurance companies and the lawyers. To accept anything like that counts as payment, which could still be insured but as a buisness at much higher costs. Can you imagine a farmer paying higher insurance premiums to cover a friend arrowhead hunting on his place? Thats in Iowa anyway but other stats are probably simmilar. Kind of messed up i think.
I suppose there are a few bad apples that do something dumb and get hurt and sue the owner and ruin it for everyone. Its like spilling hot coffe on yourself an suing mcdonalds for selling it to you.