Grab a chopper from the heli tours pad in Algonquin and carefully fly underneath all of the main bridges that pass over water in the game. Below is a map showing which bridges these are. You will see an on screen prompt letting you know when a bridge has been passed under successfully so if you don't see one, you need to do it again. Take extreme care going under the low bridge going from Bohan to Algonquin as there is very little margin for error. Your best bet is to approach the bridge then bring the chopper down to just above the water and steady it. Once you are practically stationary, very gently tilt the
button forwards to slowly crawl under the bridge.