The lighthouse in III can be reached via Dodo or Speeder...I don't know of any other vehicle to work (at least, without involving cheating in any way).
The Dodo can simply be crash-landed there. The potential problem here is that you have no control of it as this is occurring (so you have a high chance of flipping into the water instead of being able to step out onto the rock). You can take advantage of the "Taxi Driver" glitch to soft-land the Dodo instead:
- Use a nearby taxi to trigger the mission, and switch with the Dodo as the mission key is still held down. - Pick up your initial fare that is reasonably close to the lighthouse and has a good area to use as a runway...the long cement Northern border of Portland or under the loading cranes at the shipyard work well. - Before your fares' timer expires, fly over the flat portion of the lighthouse rock and cancel the mission. This stops all movement and the Dodo will float directly down onto the rock to let the passenger out (allowing you to do the same if your wheels are level).
Using a Speeder is even more trial/error. Look for the small triangular texture piece of rock right at the water surface on the rock. This is visible on the Northern face...directly in the center of where the flat surface exists above (i.e. your intended destination). Go a fair distance away and ram this spot at top speed when the water is calm. If all goes well, it'll push you right up on top of the rock itself.
In any case, staying there is not an option. The game engine will slowly push you off into the water. Obviously, using a Speeder is the only way to both land there as well as escape unharmed.