The Blue Hole (Blue Lagoon), the location for several films, is worth the trip to Port Antonio. It's a small beautiful lagoon with deep and somewhat cold waters. The water color is well worth the visit. Note that there are no facilities at Blue Lagoon.  There was once a beautifully located restaurant, which was built for one of the film sets, but this has now closed due to hurricane damage.

It's a public place, but you will see local scammers asking for J$150 per person (US$2.5) as an "entry fee". Though it's well worth to be at the Blue Lagoon, swimming is fun. While you are here, you can also check out the Rafting trip at the Rio Grande (day trip $50, night trip - moon light trip - is around double price).

It can be accessed via a narrow lane off of the A4 coast road, signed Blue Hole.  Another way of getting there is from San San beach where touts will offer to take you by boat around the corner to the Blue Lagoon, then moor up there for you to have a swim.  It's a short boat ride, passing some wonderful waterfront villas.  On the way back, the boatman will offer to drop you on the tiny white sand beach of Monkey Island and arrange to return for you at an agreed time whilst you go snorkelling to the side of the island.