Depends on whether cold fronts have made it this far South and temps are in the 40s. For the past years though, it has been almost as hot as summer in winter so plenty warm enough to swim. Ocean water might have cooled down to about 65-70 in December so as warm as the pacific in California or as the ocean is all summer in Maine or Ny.
Atlantic beaches to check out.
New Smyrna, Ponce Inlet, Cocoa Beach, Melbourne area Beaches, Sebastian Inlet State Park, Vero Beach. Just Google for pics and info on the beaches in each area.
For a natural rather rustic beach check out the National Seashore near Titusville. No food, electricity or running water there, but there are port-o-p0tties inside block buildings at each parking access.
If you want a boardwalk with rides, games etc. , Daytona Beach is your best bet. cocoa Beach Pier has bars/restaurants and shops.
Ponce Inlet also has a light house museum to see.
I don't really have a favorite , just like them all.
Gulf side beaches are a totally different experience. Beautiful but sort of boring at the beach if you want to try surfing or boogy boarding since you won't get the wave action of the ocean unless it is storming.
If you want shells you won't find as many as on the gulf coast islands on the east coast. There are shells on East Coast but not many in Daytona/Cocoa Beach areas. South Brevard seems to have more because of the sand bars. If it is a full moon time and you are there during low tide, you might even get to see the our reef and some cool creatures like turtless at some of our beaches.