It is not unusual to find fossils from land creatures and fossils from aquatic creatures in the same area, though they generally come from different times.  The southeastern U.S. is one of those locations.  It has been above or below the surface of the water many times.  Florida in particular, is very flat and close to the current sea level (as is much of the eastern seaboard).  It was completely underwater during the Ancient Warm Earth (left map below).

One thing that effects sea levels is an Ice Age.  From a geological standpoint, a single Ice Age is not that long an event.  But they tend to come several at a time, in a series, like the temperature line below.  Imagine that each downward curve represents an Ice Age, with intervening times, where the Earth warms back up.

There have been three times when the Earth experienced a major series of Ice Ages.  And of course, there have been lesser series at other times, just to clarify for all you "got to be exact" folks (you folks are probably cringing as you read this simplification anyway).  The first major series of Ice Ages was about 700 million years ago, when life was little more than slime.  The second was about 300 million years ago, when plants and fishes ruled.  The third series of Ice Ages is the one we are experiencing now, and started around 4 million years ago.  Between the middle series of Ice Ages and this most recent one, something dramatic happened.  Animals crawled from the oceans and flourished on land.  First the amphibians, then the reptiles, then birds and mammals.  And you can imagine how many fossils we've left lying around during those millions of years.

As this current series of Ice Ages has played itself out, sea levels have changed dramatically, which has influenced what animals lived where, and thus what fossils are found where.  At the peak of an Ice Age, glacial ice extends from the poles, covering land and depleting the oceans.  Think "ice" - "frozen water" - "less water in the oceans" - "lower sea levels."  Look at the map below on the right.  During these very cold times, Florida expands and becomes the southern-most refuge for land animals moving south to escape the cold.  You can see now, why fossils from land animal are found off-shore.  Between the Ice Ages, when the earth is warmer, the glacial ice melts, retreating toward the poles and dumping water back into the oceans.  Sea levels rise significantly, perhaps as high as they were during ancient times (left map below).  At this moment in time, we are between those extremes (middle map below).
   

   
So, between Ice Ages the land is underwater and the fish swimming above deposit their remains (fossils).  At the peak of an Ice Age, land creatures are there depositing their remains (fossils).  When you dig, you find a shark tooth right next to a mammoth tooth.
 
As an interesting side note, the Megalodon shark teeth found in Florida tend to be smaller - generally around 2" to 2.5".  It is believed that Florida, being shallower water, was a "nursery" area for Meg.  Exceptions to the rule do rarely occur though, such as the tooth to the right.  It is one of the nicest large Meg teeth I've seen recovered from our area in Florida.  It measured nearly 4", was in wonderful condition and has amazing "marbled" coloration.  Most Florida River Meg teeth are gray in color.  Occasionally you will see tan/brown tones or black.  Land found teeth tend to be much lighter in color, often with a white/light root and a blade that is blue, black, orange or some other interesting color.  In fact, the name of the entire fossil formation "Bone Valley..." in many people's minds "means" the lighter colored land-found teeth.  They generally come from Shell Pits (concentrations of fossil sea shells - used like gravel for roadways in Florida).

The southeastern U.S. location where the largest Meg teeth are found (on a regular basis) is the Hawthorne Fossil Formation.  It is located on the Atlantic seaboard of Georgia and the Carolinas.  Teeth are recovered off-shore, from the inland waterways and most commonly from the rivers that cut through the formation.  Because of the size of teeth found, one could assume these were adult "feeding" areas.  You see, feeding is what produces most of the teeth we find.  When a shark dies, he takes with him only a hundred or so fully developed teeth.  Throughout his lifetime though, he'll shed thousands of teeth.

South Carolina's Cooper River, where many of the largest teeth are found, generally produces teeth that are gray/charcoal/black in color.  Georgia Rivers generally produce teeth that are more brown/rust in color.  North Carolina and Virginia rivers often produce teeth with brown/rust/red colors.  Land sites often produce much lighter and more interesting colors.  In North Carolina, the PCS (Lee Creek) Phosphate mine exposes the Yorktown and Pungo Formations, producing light gray/tan and dark gray/black teeth, respectively.  And the Megalodon teeth from there tend to be smaller (like Florida).