This is the best way to get to Mona. The trip takes about 30 minutes and its a great way to see the Derwent river. The wharf is located on Franklin Wharf and it costs about $20 return per person.
You can purchase food and snacks on the way over to Mona. Just let them know if you have any issues climbing stairs when you get to Mona, as you have to climb up 99 steps to get to the top. There is another wharf that they can drop you off at and the staff will drive a buggy down to collect you, so you don't have to take the steps.