There are three ways to get to Santiago Atitlàn: private boat hire, public ferry, or a tour. We chose the public ferry which is the cheapest option and takes about 30 minutes from Panajachel. The fare was 25 Q per person each way. One advantage of the ferries is that you don’t have to buy a return ticket, simply turning up at the dock when you want to go back.
Bear in mind that the ferries don’t run to a fixed timetable but leave when full, so your overall journey time could be quite a bit longer as you also need to allow waiting time while the boat fills up with passengers. We waited almost 30 minutes on our outward journey, but on the way back we were the last passengers to board and left immediately.
A private boat on the other hand will leave when you want and return at the time agreed between you and the boatman. This means you need to be able to plan in advance how much time you’ll spend in the town, and with so many shops to tempt you as well as all the interesting sights, that could be hard to do.
The third option is a tour. The plus side is that a guide will take you to Maximón (no need to pay the local children – although personally I felt that they probably needed the money more than any guide!), and explain fully what you are looking at. There was a small group there when we visited and, rather cheekily, I listened in! The downsides to a tour are the extra cost, and the limited time you will have in the town – most tours visit three villages in one day so you may feel rushed. The tour is possibly a good option if you only have one day in the area, but otherwise I would recommend the “do it yourself” approach.
Head to the lakeside in Panajachel and the boatmen will find you. Remember to barter if taking a private boat, but the ferry fares are fixed (and are dearer for tourists than locals, so don’t complain if the man next to you is paying less)
If you're staying in another village around the lake you might take a ferry to get to Santiago Atitlan. We stayed in Pana but used the ferry to get to Santiago. One of my lasting memories of our trip was the young man yelling out "Panajachel, Pana!" in an effort to attract more passengers for our return ride from Santiago Atitlan. The ferry is really more like a launch and depending on the size will hold from 20 to 40 passengers. The fare is a whopping 10 quetzales (USD 1.30) between Pana and Santiago Atitlan. The price will vary depending on from/to where you are coming/going.
Be aware that the rides can be pretty rough in the late afternoons if the xocomil (wind) kicks up.
It's highly likely that you'll arrive in Santiago Atitlán by boat like I did. A lot of people visit as part of a boat tour that usually takes in San Pedro, Santiago Atitlán, and San Antonio Palopo. This tour, which departs from Panajachel, should run you around 70 Q or less depending on how good you are at bargaining. Alternately, you can catch a boat directly to Santiago Atitlán, a ride which should cost under 15-20 Q.