When you say you've been staying on top of watering, what does that mean? Are you watering more frequently than once per week? Because the grass appears to be diseased from being too wet. Here's a close up of the traditional St Augustine disease.

The tan spots with dark brown rings is what you're looking for. You may have this or something else, but I get this every year in one spot or another. If you have Floratam the grass will outgrow the disease and look okay, but not excellent. If you have another slower growing variety, it will thin out all the way to nothing. My picture was taken on a cloudy day from a distance of 6 inches away. If you can post one like that, we can see it much better.