There is a ferry between Breskens and Vlissingen, running hourly at least and more in summer, during the school run and more.
You can only use that ferry by bicycle or as foot passenger, cars and buses have to use a tunnel farther away, there is also bike transport through the tunnel as cyclists are not allowed to ride it.
Here is a map of the area with the ferry and the tunnel on it.
So find yourself transport from Antwerp to Breskens, join the ferry across and take the train or an other bus for the last stretch.
As alternative, go Dutch, get a bike somehow and cycle the whole distance.
In Belgium you can use 'de lijn' for buses and in the Netherlands OV9292 for all public transport, so uncheck the trains in '+ more options'.
Check routes for a series of border towns, local buses do not cross borders as a rule, and long distance buses do not run in this area.
I have done that search before but so long ago that I will not find the (Yahoo!Answers) information anymore.