Never been there myself, but have heard good and bad things about it, Here are some comments I found:

"SHELL CREEK -- the name says it all. In addition to ancient shells, you can find Paleo-Indian spearpoints in number, and huge Megalodon teeth that rival any location in the world in quality. There is a boat ramp in a fish camp on the W. end of the Hwy. 17 bridge that provides access. To get to the bridge, turn off Hwy. 75 in Punta Gorda, and go E. on 17, until reaching the bridge. The fish camp will be on your left before you cross over."

"There are several tributaries to Peace river, some of which are supposed to be good for fossil hunting. I've tried Horse Creek, Prairie Creek, and Shell Creek, and all seem pretty worthless. I've heard that I may not have gone far enough upstream on Shell Creek to get to the fossils, but it looked unpromising where I was kayaking. Matt Mattson from treasuredivesites.com says that some big Megalodon teeth have been found there, and I have a lot of respect for the man, so maybe you'll have better luck than I did."