I would strongly advise against drinking ‘a couple of liters’ of tonic water, for two reasons:
1. The quinine content is quite low - it is not legal in the US to have a higher concentration than 83mg/l. A therapeutic dose would require at least 200mg twice a day - which translates to around 5 liters of tonic water (that is one of the COVID-19 treatment protocols - malaria requires 500mg - 1000mg per day).
2. There is a lot of sugar in tonic water - around 10g/dl. So if you drink 3 L of tonic water per day (which would only give yourself a low quinine dose), that would be the equivalent of about 300g of sugar.
Drinking 5 L of tonic water per day would not only increase the sugar intake to 500g per day, but would also potentially overload you with water, which can be lethal, depending on your underlying health.
Admittedly, if you were drinking tonic water with chloroquine as a prophylaxis (prophylaxis for what? - malaria? SARS-CoV-2?), you might be able to get away with a smaller dose, but there is absolutely no data available for that.
So by ingesting the amount of sugar you may well ‘upset the territory’ more than you gained by ingesting quinine.
Further I would question the wisdom of taking quinine as a prophylaxis. Hydroxychloroquine seems safe and effective for the treatment of Covid-19, IF you are symptomatic. Given that getting sick of Covid-19 if infected is either unlikely if generally healthy, or associated with a mostly mild to moderate illness, I would think twice about using chloroquine as ‘prophylaxis’.
There are, I think, better alternatives to boost the immune system to make it stronger against viral infections: make sure your vitamin D levels are appropriate, vitamin C, zinc, iodine etc.