You can do this very early on in the game, but it is recommended that you have unlocked the safehouse where the character is located in the screenshot below: (The bridge is marked using the waypoint). It will also help to have some Agility upgrades in order to make your way across the bridge more easily, and you should have a medkit available just in case you need it.
Once at the safehouse above, climb up the radio tower and use the bed to fast forward to night time. With the bridge marked on your map using a waypoint, you'll want to come down the tower and go for the bridge. Make sure to stay on high ground so that you don't end up under it. Once on the bridge, just keep running forward until you reach the end and then sprint off to fall into the water below. Make sure to not jump off the sides or you may hit land instead of water. It is best to avoid the volatiles that will show up on your mini-map and you will probably want to use cars and buses on the bridge to stay above the zombie horde and avoid a fight with them. Once you reach the end of the bridge, run off and it will unlock when you hit the water. You will want to swim to shore and head back to a base if you want to keep your survivor points.