Several operators in San Pedro rent sit-on-top kayaks and stand-up paddleboards by the hour or the day. The lake's volcanic water is remarkably clear, and paddling gives you access to quiet stretches of shore that are impossible to reach on foot or by lancha. The most popular route is the short crossing to San Juan La Laguna to the north — close enough for a half-day, with good swimming spots along the way.
Go in the morning. By early afternoon the Xocomil, a notorious wind that builds from the south, can turn the lake rough with little warning. Kayakers and paddleboarders have been caught out by it; the locals' rule is to be off the water by noon if you are not experienced. Equipment rental is cheap, typically a few dollars per hour.