Jennifer Lopez is drawing a line in the sand -- paying nearly a grand to rent out a public beach in L.A. last weekend ... to keep randoms away from her family and children.
Sources tell TMZ, Jennifer forked over $925 to the city of Long Beach to rent out Marine Beach on Saturday -- 3,400 feet of shoreline -- just for her, her kids, her BF Casper Smart and other family and friends.
To keep out the riffraff, we're told lifeguards marked off the water using buoys -- and kept the beach gates locked.
We're told J.Lo and co. brought their own jet skis for awesome fun times -- and even hired a wakeboard instructor.
Meanwhile, a bunch of sad locals stayed home and played with dirt.