there are many places. But it depends on where you want to go. ( to save gas etc). You can launch in mission, near the pub and dive shop, you can also go to Kilbey ) over on the way to Harrison) you go into the old kilbey historical site and launch into the Harrison, just travel down river to hit the Fraser...
You can enter near the Vedder ( bottom end) at the Barrow town pump station, and you can also find many spots along the mission side... go dont the ***** and you will find entrances all over the place.
But, as mentioned before.. Island 22 is more secure than other places. I have come back to find my truck had been opened.. but nothing taken.