There are several of the loveliest beaches in Jamaica between Long Bay and Port Antonio. There are actually many more beaches around the area, but here is a sampling:
 

Blue Lagoon    (click to enlarge image)

It's not really a beach, but it's a great place to swim and relax. Famous 180 ft. deep lagoon with natural hot springs and spectacular scenery. Go snorkeling or use the lagoon to get in some laps, the beauty is tough to beat!  The Blue Lagoon, "Blue Hole" to the locals, is fed by hundreds of natural mineral springs.  David Lee, son of the local dive shop (Lady G'Diver) owner, made his record-breaking free-dives in the Blue Lagoon.  Although some of the locals claim there is a sea monster that lives deep within the Blue Hole, the famed oceanographer Jacques Cousteau claims to have dived down to
200 feet and found no bottom and no sea monster.
This is a private swimming hole, no fee.

 

Boston Bay Beach

Home of the famous Jerk-Chicken and Jerk-Pork. Take in the gorgeous beach and some great Bar-B-Q food with roasted bread fruit and cold Red Stripe beer, a divine beach picnic.
This beach is public, no entry fee.

 

Dragon Bay Beach

Resort villas and great reef snorkeling. Dining, dancing and refreshments are also available.
This beach is private, no fee.

 

Frenchman's Cove Beach    (click to enlarge images)

Stunning scenery surrounding this private inlet beach.  A favorite of Jamaicans and visitors as its clear waters and stretch of sand invite swimming and sunbathing, and local cuisine is served directly on the beach. This is one of the prettiest beaches and often features on the Jamaica Tourist Board promotional material. The beach is flanked by lush vegetation and a cool stream winds its way to the white-sand beach. Outside showers are available to wash off the sand in addition to the fresh water stream. Changing facilities are also provided. You can rent lounge chairs and have lunch served at table. 
This is a private beach, entry fee is about $2 US.
 

Long Bay Beach

The Long Bay Beach is only 3 minutes walk from Blue Heaven.  1 ½ mile virgin white sand beach.  Long Bay Beach is the top surfing and body surfing beach in Jamaica.  Long Bay beach is highlighted with a beach cafe, Chill Out, offers a variety of dishes and cold drinks for you enjoyment on the beach. 
This beach is public, no entry fee.

 

San San Beach

Great for swimming, snorkeling and wind surfing, the water is warm and shallow.  There are some reefs a few yards from the shore that offer good snorkeling. Just across from the beach is the beautiful Pelew Island, also known as Princess Island, which is also great for snorkeling or just lazing if you are up to the swim there.
This is a private beach, entry fee is $8 US.

 

Winnifred Beach

Also known as Fairy Hill Beach, this is a fabulous public beach with a nice beachside eatery.  Cynthia and Painter at Winifred Beach offers up some of the best lunches around, including authentic and renowned Jamaican jerk dishes. 

One of the biggest and most appealing public beaches. The wide, golden crescent of sand is popular with local citizens and visitors looking for an informal, lively setting.   The small reef just offshore is perfect for snorkeling (bring your own gear) and protects the bay from the waves ensuring clear, calm, bright-blue water that shelves gently from the sand. At the eastern end a small mineral spring offers a fresh water rinse. 
This beach is public, no entry fee.