Hi Liam
There is no ferry service to Saibai. Flights via the small air strip are limited and usually for formal visits to Cairns, the mainland and locations that are a geographically distant, such as Thursday Island.
Torres Strait Islanders are a sea faring people. For the majority of the people, they freely travel to other island communities including PNG via small boats, occurring often for cultural reasons, trade etc. PNG visitors often stay on Saibai during visits.
For travel to Thursday Island, you can assume that Saibai Islanders do not travel directly, but rather, divide their trip based on stopover Islands along the way.
Hope this helps