There are 12 headings that will allow someone to leave the Island, one for each of the 12 Vile Vortices. Michael's raft and Desmond's sailboat couldn't get away from the Island because they weren't following any of the proper headings. You can enter and leave the Island normally by following the bearing that corresponds to whichever Vortex the Island is currently in. Following any other bearing will take you to another Vortex. The bearing that Ben gave Michael (325) probably sent him to the Bermuda Triangle. He would have been back in the USA within a day.
Or Ben and the other do not know the actual bearing, but use it and it does not effect the time enough for them to notice.
The bearings to the island changes after every purple sky event.
Michael followed a 325 bearing after the Swan discharge. The sky turned purple, then Ben gave him the bearing and he left. Maybe 325 leads straight to Ben's people, but the freighter crew never knew about that one, just the other safe bearing, 305.
As Desmond says that they 'are stuck in a snowglobe', the only way out of the snow globe is the 325 degree bearing.
The Black Rock (or some other ship that crashed on the Island), made created a hole in the "snowglobe.
The electromagnetic anomalies found on the island interfere with compass readings (Sayid has already pointed this out) and as such will guide anyone navigating via a compass back around to the island. In "Live Together, Die Alone, Part 1" it is confirmed that the Electromagnet is destroyed or useless, so the natural anomalies it controlled are now present again.
This would not affect GPS, radio compasses, navigation by constellations or sunrise/sunset. Magnetic anomalies are insufficient to cause this level of navigational difficulty.
The compass bearing may be invalid, because it is based on data obtained prior to the discharge.
Bendoes seem rather unimpressed by the unusual events, may be familiar with them, and the bearing he gives may be the only one which can withstand the magnetic fields which now emanate from the Island, and leads to an escape from the Island.
In an interview with Michael Emerson, who plays Ben, he said that his character was not surprised to see the sky turn purple, so he at least knew something about what was happening.
Since connection to the Numbers is typically bad luck, 325 having no relation to them may mean it actually is the only way off the Island.
Such headings are not always followed exact. If the actual heading is supposed to be 324 (3x108) it makes sense that Ben (and the writers) would round up or down to the nearest five. Michael and Walt would still see the same land mass or 'exit' as on a 324 bearing. One degree wouldn't make all that big a difference.
Though, depending on how far you are traveling, any pilot or sailor will tell one one degree can make ALL the difference. Also of note is the magnetic declination of a given point on earth
Location of the Island
There are very few places in the South Pacific Ocean where a heading of 325 would intersect with another land mass within the potential range of the boat. Either the Island may be located within a hundred miles or so to the south-east of a more civilized real-world location.
Connection to the Swan Hatch Mural - The arrow in the middle of the mural is pointing at 325 degrees and has the word "Out" next to it.
The bearing was calculated by the mathematician Horace Goodspeed, who worked in The Arrow (this was perhaps the only purpose of the station, which explains why it was later used as a storage facility).
The news report of the 'discovery' of 815 on the seabed mentions the loss of 'all 324 passengers and crew'. Assuming that Aaron is not included in this tally, his inclusion would bring that total up to 325.
Daniel gave Frank the correct vector, but due to the storm they veered ever so slightly off course to about 310 (reading from the gauge during the storm).
In Live Together, Die Alone, Ben tells Michael to leave the Island and follow a bearing of 325 degrees. However, Faraday insists that the helicopter must follow a bearing of 305 degrees to safely exit the Island. The bearing at which you enter or exit the island is important. The Two bearings could both be courses to get off the Island if there is some sort of "keyhole" departure/entry point for the island. Since the helicopter and Michael departed from different points on the island the two bearings could intercept at a given location (i.e. the thunderstorm) which allows vehicles to enter or depart from the island. Like two corners of a triangle, each bearing moves in different directions (their sides) to reach the third point.
The bearing is 305 from the survivor's beach, but 325 from the Others' boat dock further north. The bearing is to a fixed point (the storm), so the bearing has to change somewhat if you move up or down the island's coast.
If the Island is in fact surrounded by some sort of field, and this field revolves around the island, it would explain the fact that the location of an opening in the field changes depending on what time it is.
The Island is in some sort of "alternate time thingy" and it is located at the North Pole (Google the book The Smokey God and/or the manuscript "Admiral Byrd's Secret Diary" and/or "Hollow Earth Theory").
It is possible that the bearing changes, so that 325 was the required bearing at that time, and 305 now. If this is true, then either the bearing changes are predictable or the pathway can be detected since Ben knew the bearing.
325 could be a salute to the Alfred Hitchcock film "North By Northwest" (1959) which involves a case of mistaken identity and a 3,000 mile chase. Alfred Hitchcock had originally intended to film "The Wreck of the Mary Deare" (1959) which involves a small rescue boat encountering a mysterious freighter left adrift.