Jennifer Lopez Rents L.A. Public Beach -- Keep Away From My Kids

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.