If your intention is to go to Santiago, when you arrive to the pier of Panajachel you will get a lot of offers. If not do much, because all will be for private transport. Find schedules of either the public boat (also tell you that no more services until the next day) and wait for departure. Much cheaper. And more interesting, by sharing the premises with the voyage. It also monitors the weather, because the wind gets up a treacherous wave.