There is a cafe on a few of the ferries but a toilet is not mentioned.
Are there toilet facilities on the Stradbroke Island ferry?
If not, is there one at the ferry station - one that can be used before getting on the ferry?
Based on this August 21, 2016 review
What's the go with the same price to have no access to a bathroom, no viewing deck and $10 to change the time (not advised of this charge when booking, might be a good heads up) - if your going to take away some of the comforts you ought to take away some of the price!
There seems to be no bathroom on the ferry.
According to this review however,
The Cleveland office also has a good little cafe and toilet.
If you are talking about the car ferries, than yes there are toilets on them (even the older models). Look for signs upstairs in the cafe area.
Source: am local and have traveled on this ferry many times.
All ferries are made with a toilet. For you curiosity you can use toilet before getting inside the ferry so that in case toilet is missing, you will be Ok