You dont really have to "unlock" them. You can just play freely.
with the tri-pass you unlock islands 1-3. with the rainbow pass you unlock islands 4-7
level 49 is when you unlock the m8al in bo2
You have to get the Master ** Rank.
49/49 not including recon.
you have to get 49+ acheivments
Unlock all 49 achievements
You unlock other islands by completeing other islands.Sometimes the map is given by the person you help.If you fill up the king points gauge the king will send you maps which let you access other islands.
When you unlock create a class.
defeat Blian
beat some missions and then u will unlock 1 by 1
birth island and navel rock and eight and nine islands.
If you are talking about Vice City ,I unlock all the islands after mission 13.