There is no access to the beach at the 12 Apostles - the stairs that used to be in place are now affected by erosion and are too dangerous to use.
The currents in the water are very strong and I think it would be impossible to get there by boat. The waves on the beach would make it impossible to land on the beach in any case.
As for doing it with just a paddle: not a hope!
If you want somewhere interesting to go down in the general area, try Wreck Beach.
http://www.the-great-ocean-road.com/wreck-beach.html Lots of debate about how many steps it is - the signage indicates somewhere around 350 - 400 stairs but there are many added notes from people whose counts differed, and in any case much of the track in is steep without steps. It takes a bit of dedication to get there.
Audrey