The Manasquan River is a major waterway in central New Jersey. It flows from central Monmouth County, beginning in Howell Township, to the Atlantic Ocean, where it empties between the communities of Manasquan and Point Pleasant via the Manasquan Inlet. It widens greatly as it nears the ocean, making it ideal for boating, and is a very popular recreational area of the Jersey Shore. The Manasquan River and Inlet is the northernmost terminus of the Intracoastal Waterway.

Situated in the middle of the river east of the Route 70 highway drawbridge and adjacent to the former Point Pleasant Hospital site and Point Pleasant Canal entrance is a sizable island only accessible by boat that is a popular spot for picnics and recreation with boaters. The island includes a sandy beach area and even an improvised swingset for youngsters, but is mainly covered with trees. The water surrounding the island is deep enough to accommodate boat traffic, and boaters normally beach their boats on the sand while they enjoy a day of relaxation on the island.  One day Robert Lewis Stevenson visited Osborn Island and was so impressed he whimsically re-christened it "Treasure Island" after his famous novel Treasure Island (1883) and carved his initials into a bulkhead. This took place five years after he had completed the novel. To this day, many still refer to the island as such