The other thread disappeared as I was explaining this for the benefit of others going to Venice.
You CAN safely board a vaporetto without a ticket at the docks where there is no ticket machine. We have done it many times. You must inform the crew as you step aboard, not later.
I look him in the eye and say "non abbiamo biglietti/we need tickets" as I step past him and wait for him to acknowledge. Then I wait near him as boarding is completed. He will then go and get his machine and sell us the two tickets.
And by the way, even if you are using a pass for the vaporettos you must validate it at the machine before entering the boarding area. At the docks where there are ticket machines or sales agents, it is a violation to even be on the dock without a validated ticket or pass. There are signs up telling you that.