This question has been one of great interest for me as well.
Though not particularly helpful, this is what I have been able to gather from different sources...at least with regard to
saltwater beaches and estuaries but not including lakes or ponds.
The mean high tide line has been the accepted beach boundary between public and private for a while. Just where
this line should be delineated is apparently up for extreme debate. Scientists seem to agree however that this average
level of high tide mark has to be computed using data over the previous 19-year lunar cycle. Scientists quoted in a
recent sarasota herald-tribune article (see saturday may 13,2006 archive) say that it is almost impossible to compute
this "mean high tide " mark. For example, If you tried to compute the mean high tide mark for Ft. pierce, Fla, you would
have to factor the huge surge levels caused by the two major hurricane landfalls in that near vicinity in the last couple
years. This would make the mean high tide mark alot higher in Ft. pierce due to these storm surges than in some place
where no hurricanes or storms had come ashore. the herald-tribune article quoted some local police who said they had not
issued any citations or arrests in 18 years concerning beach trespassing because there simply is little hard evidence that
would hold up in court concerning this type of trespass.The article also said....well screw it..I'll stop being lazy and
unscholarly and find the thing!......dammit...it's gone to the paid archives..well..maybe one of you out there has access to paid
archives...it was in the letters to editor section of the above edition of the sarasota herald tribune, may13 06. the artcile went on
to quote the sarasota police as saying they only enforce when someone comes really close to private property as in being almost
in their backyard.
Personally, since the beach is part of a large littoral system which begins off shore and ends in
the dunes inland, sometimes quite a ways inland, i will be detecting and walking on any beach as long as I stay off the dunes!!...