To find your way to Certosa Island by waterbus (Vaporetto) you easily use the App/Website “Chebateo?” Where you type in your place of departure (Waterbus Terminal) and Certosa for the destination.
The Vaporetto 4.1 and 4.2 will bring you directly to Certosa without changing the waterbus. You can quickly check the timetable here:
Please remember Certosa stop is only on request: tell the inspector that you want to go to Certosa.
If you need help buying the Vaporetto tickets, you can find some useful information and tips here.