Calle Santander 3 Av. 3-47, Panajachel, Guatemala
As soon as you get near the lake, you will be accosted by a swarm of people trying to get you to go with them to cross the lake. They tried to tell us that the public ferry cost $100 US to go round-trip and they said they would take us for only $70 per person.
Unique Suggestions: We had a Guatemala guide so we told them they were lying and the public ferry is only $2.50 RT and then they dropped their offer to $10. We ended up getting a ride for only $7.
Fun Alternatives: If you don't want to go on a private boat, you can always take the public ferry but it's extremely crowded and leaves on a (supposedly) set schedule so you might have to wait a while.