Jennifer Lopez Rents Public Beach For The Day

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.