From San Jose, you can fly to Bocas del Toro on Nature Air, or you can take busses to the border and then to Almirante. From Panama City, you can fly on Air Panama or you can take a direct, overnight bus to Almirante for something like $38. From Almirante, you take a water taxi to Bocas Town and then another water taxi to Isla Bastimentos.
Do you have to take a boat? Is there any way to drive there?