To get to Panajachel by tourist bus or shuttle, they leave from the main street of Panajachel. If you want to go for a walk around the lake and other towns, you will have to travel by boat. Tourists are easy scam targets by local boaters and other vendors: this means it's not worth buying a private trip when with some patience the public boat will be much cheaper and just as fast. Just do not think they are cheating if everyone does not pay the same price: the "local" inhabitants of the lake, pay a different amount. For "non-local" tourists or Guatemalans, it's 25Q, but it may have risen to 30 since last spring.