The sand leading down to Inskip for boarding the barge is quite soft, so ensure your tires are deflated to a maximum of 20psi. The barge service is frequent, with two barges alternating from side to side. The queue to board is usually short. If you encounter difficulties in the soft sand, you can flag down help as people are generally willing to assist. There's no need to purchase tickets in advance; you can buy them on board the barge.
You don't need tickets to enter, so I'm not sure why that option is available. However, if you're driving around Rainbow Beach, you should definitely check out all the viewpoints. When we visited, it was just us and another family there. It's very easy to access, but since we wanted to spend more time on Rainbow Beach, we didn't stay long. The views are stunning, and if you're looking for a quiet spot to enjoy the sand and sea, I highly recommend it!