As you can see lemon juice is something that can adversely affect your enamel (1). Acid affects your teeth because it leeches calcium from them (2). When this happens your enamel breaks down making your teeth are more vulnerable to bacteria and plaque which then leads to decay. Pure lemon juice is not something you should be downing constantly throughout the day if you want to protect your teeth. That being said it probably isn't enough acidity to affect your stomach enough to cause acid reflex.
However you are drinking very dilute amounts of lemon juice which probably makes the effects negligible. I can't find any research to support this (there isn't much on this topic) but unless you are also drinking something very acidic on top of the lemon water I'm sure you will be fine.
Just to keep an eye out you should look for signs of tooth decay, here is a good place to check for signs/symptoms. However ignore the advice that "all carbonated drinks" harm your enamel. This is simply not true since not all carbonated drinks contain high amounts of acidic compounds. However, this is true of all soft drinks (aka a "soda").
To ease your mind, lemon juice with mineral water is a traditional drink in Italy and across most of Europe. I personally have grown up drinking it, along with all of my relatives, and have never experienced any enamel weakening. In fact my dentist regularly comments that I have incredibly strong teeth.
Still this simple breakdown of highly acidic beverages may help you avoid damaging your enamel too much, if you are worried of such things(here).