One thing to note is that the harbour at Mersing is fairly shallow. This isn't a big problem as such, because the ferry can still pass in and out. But it is a good idea to remember that ferry sailings usually take place at or near the high tide marks.
As a result, departure times to Tioman and back to Mersing tend to be different every day. Unlike most other ferry operations, the ferry to Tioman and back to Mersing doesn't have fixed sailing times.
As the Mersing tidal ranges are more extreme during the monsoon months, November to February, ferry times during this season vary the most.
But even during the rest of the year, scheduled ferry times are still subject to amendments, according to changing weather or sea state conditions. It doesn't happen frequently, but it does happen, so it's good to be aware of this.