The water is perfectly safe to drink. It's more trustworthy than the water in my city back in the States. The air quality is not good at all. If you go by Plume, you will see what days you should engage in outside activities. The ride from the airport is a little wild, so Dramamine may be useful. Uber is illegal, but it's the best way to not get ripped off by your driver (taxis used to get me all the time), just make sure you sit in the front. Be sure to check your route once you're out of your Uber to make sure you didn't get scammed as well (this has only happened to me twice). Your trip should end when you get out of the Uber. Good neighborhoods for foreigners include the following: El Poblado (expensive, good night life), Laureles (more authentic, good street night life), Envigado (farther away from some cool areas, but very chill, more authentic, cheaper), Guayubal, and more. No matter how good the AirBNBs downtown look, just don't do it. Don't be on your phone outside at night. Put it away and you'll probably be fine. The internet is fine! I work every day on the internet and have never had any trouble in any neighborhood. Food can be very cheap outside of El Poblado. Other than the amazing fruit selection, the food is bland in most restaurants. Spice is not really a thing here. Last thoughts: cars will not stop for pedestrians, go salsa dancing every night, and you can find a cheap gym - just keep looking!
I was there for one week and stayed in Bocagrande, about a 10 minute taxi ride from the Walled City. Pros of Cartagena: Inexpensive locale, reliable and fast internet, good weather (if you like it hot), not terribly crowded (aside from Walled City), beauty/history/nightlife of Walled City. Cons: Local beaches are average at best and riddled with vendors who accost visitors non-stop, street vendors are less pushy but everywhere, fluency in Spanish is very helpful (not really a con, just is), Walled City is loaded with tourists and priced accordingly. Cartagena struck me as a one-trick pony like a Las Vegas. Some people love Vegas because of the weather and the entertainment. Cartagena has the weather and the Walled City. Unfortunately even though its a coastal city, it's tough to enjoy the beach.