In Pennsylvania, my understanding of the law is that the property owner owns the streambed under the water and any dry land - stream banks, islands, exposed sandbars, etc.  However, if at any time in history, the water has been designated as navigable, you are allowed to float or wade.  You may be prevented from stepping ashore by the owner, but he/she cannot tell you to "walk on water".

There's been a bit of controversy over this topic in PA, as it seems there is controversy everywhere - with some stories of PA landowners stringing cables across streams that are paddled by kayakers and used by trout fishermen - a rather dangerous situation potentially.  Unfortunately, I can't recall if I ever read about whether or not the landowner is allowed to do such a thing and if that cable (or others like it) are removed once the authorities become involved.