In my mind (mostly my imagination) the patterns on those agaves might have to do with irregular growth patterns. Any given leaf may stop and start (based on water availability or some kind of natural program) as it extends to reach full length. In the process the production of the waxy farina is somehow not well coordinated with growth, resulting in an accumulation during those pauses or upon stopping or restarting. But that is just a guess. I really don't know why it happens, or if there is anything we do as gardeners to promote this curious pattern. If water has anything to do with it, the mechanism would relate to how water availability governs the starts and stops. I am guessing the watermark term is just an analogy to something visually similar that may happen with paper and ink.
