You go to one of the bottom two bridges (cant remember which), starting from the left side of the map. jump in the water and swim to the other side of the bottom of the bridge. there is a ledge to climb up onto, but you cant fit in the space above. keep climbing up until you get stuck there with your head in the bridge. keep moving and jumping until you end up under the bridge, where you can look up and see the people and cars above you!!! you can only kill people with a flamethrower. to get out go to one of the ends of the bridge and jump out. one hole to get out leads outside the bridge, while one leads to under the land where, if you swim out too far you start to "fall". everything gets blurry, unless you pause it, in which case the entire background is black. but anyway, you keep falling until you land on top of the land above you.
its WEIRD... i found this myself by accident, so yeah. also, another thing that i could only get to work twice, never again after that, was: i got a fairly large boat and crashed it between a rock and the island where the golf course is. when i tried to jump out, i fell into the water, but the boat pushed me into the island and i was underneath, where the whole "falling" thing happened as well. i did on purpose once after that, but never again. (the second time, i blew up the boat after i crashed it, then did the glitch.)