From Sal, the only way to come to other islands is by plane.
Ships or Ferryboats do not exist in an acceptable frequency and they are really not recommendable.
From Sal to Santo Antão (not San Antao
) there is no direct connection.
First you must go by plane to São Vicente and then by ferryboat (this is one of the less ferries, they exist!) from Mindelo to Porto Novo.
Easier to reach is Santiago, which is also greener than Sal and it has a lot to discover. Be curious and take this island as a "second program" to Sal.
From Sal you can take every day a flight to Praia, but I give you the advice to book these flights ( Sal-Praia and Praia -Sal) as soon as possible from home.
I wish you an agreable stay!