ah yes, the water burn myth, something I believed up till last year. It just doesn't happen, water burn that is, so its likely the spotting on your leaf is some kind of Leaf Spot, either bacterial or fungal. A close up picture of the spots would be useful.

In case you're interested, the original 'proof' for water droplet burn on leaves was from an experiment carried out using glass droplets, the same size as water droplets. The leaves burned - but it turns out that the refractive index of glass is not the same as water, and it never happens with water droplets. I even tried to get leaves to burn myself by leaving drops on them in high temperatures (55deg C in the sun) but no luck, not a single leaf burn.