Just like most other Mexican towns, there is no shortage of churches and cathedrals in Catemaco. In my opinion, the nicest in town is the main white cathedral on the side of the main square town plaza.
Right off the Malecon, there are lots of lanchas boats ready to take you on tours around the lake showing off the local flora and fauna. I came to Catemaco looking to do this, but unfortunately due to weather it was not possible.
However I will say that there are literally hundreds of boats to choose from. The problem therein is that in all my searching, I never found any posted prices or menu boards. Therefore be careful not to be taken for too much money.