As your name suggests, You are the detailing expert, you tell me
just kidding..
There are specialised tar removers offered by most of the international product ranges (TARDIS being very popular in UK, Autoglym tar remover, Carpro Tarx is very good, Meguiars bug and tar remover and many more). They are specialised at removing tar and do so by melting it out requiring very less scrubbing (which in other instances of rubbing can cause marring). In alternates, clay also removes tar (but can end up getting clogged so not good value for money). A strong pre-wax cleanser can also help (depending on its strength) e.g. I was able to remove some tar using Werkstat acrylic strong (same like Klasse aio but stronger).
For water spots, carpro has recently launched a product called spotless that help with the water sports removal. For the ones that are etched too deep, you dont have much option than to buff it out using something like carpro ceriglass polish (or griots garage glass polish etc).
Later edit: Are you referring to something as shown here ?