There are a number of ferry routes to Salt Spring. Find what works best for you at www.bcferries.com/schedules/southern/
Please note that Ferry schedules change for different times of the year and care should be taken to be sure to select the schedule that is right for your date of travel.
Also there is float plane service to Salt Spring. Find which works the best for you.
The closest international airport is in Victoria. (use Swartz Bay ferry to Fulford Harbour). The next closest international airport is in Vancouver (use Tsawwassen ferry terminal).