Just a bit of friendly advice to fellow travellers who hate getting ripped off...
When you first arrive in Panajachel, be careful not to get conned with highly inflated boat fares. There are guys who hang around where the shuttles and buses drop visitors off. They try to bustle you along and get you to pay way over the odds, offering 'private' boats and telling you how expensive the public boats can be. It's all a big con. Don't be fooled by their official-looking T-shirts and ID badges - these men are criminals.
My partner and I are experienced backpackers but we somehow allowed ourselves to get pushed into paying 125Q each for a boat from Pana to San Marcos. Our shuttle driver encouraged us to go with these people, so he was probably getting a backhander. The guy who was hustling us told us we'd pay 25Q per town on a public boat, so five stops would add up to the same amount. When we later asked at our hostel what we should have paid, we were told that the whole trip should have cost around 20Q per person (and locals pay about 10Q). It's especially galling as we didn't like the guy and kind of suspected we were being conned.
Rip-off artists like these give Guatemalan tourism and the whole country a bad name. I hope this post enables a few others to avoid falling into their trap.