Looking at their confusing schedule, it's seems like the boat is doing:
5:30p TST 5:45p Central
6:30p TST 6:45p Central
and so on...
So, you should assume by taking the 5:30p sailing, you'll be back at TST between 6:15p and 6:30p. And it really doesn't do much then "float" in the harbor a bit for 45 minutes.
Personally, I'd buy a drink at a 7-Eleven and take the Star Ferry back and forth for cheaper.