Cartagena, a gorgeous fishing village on Colombia's Caribbean coast, has excellent beaches, a historic old town (that's entirely walkable) and beautiful colonial architecture. It's also one of the safest places in the country, so it's no wonder it's a popular port of call for cruise ships. Need a break from exploring the cobblestone streets? Stop at an outdoor cafe for excellent pastries and people-watching.
We only had a day in Cartagena, and we were able to do a lot in the amount of time we had! IN - You can opt for a private city tour, which I would recommend Cartagena Connections Walking Tour. It was the perfect start to our trip, and we were able to walk outside the walled-city into Getsamani (really beautiful graffiti) and other parts, that we may not would have known to do on our own. And they customize it to what you want to see/do/experience. Plus it's always best to get the in and out from a local.
IN - I would suggest going to La Popa, for the view of the whole area, perhaps to see the monastery itself, then head to the old city. If you have time, then you can consider San Felipe. Castillo de San Felipe if you are into history and forts. We explored it in about an hour, so it won't take much of your day. You could aimlessly wander for a while in the walled-city, and it's just so peaceful and beautiful!
It doesn't take a rocket scientist to figure out that potable water is more difficult to source in lowland tropical regions. Many Caribbean destinations warn against drinking from the tap, from Punta Cana to Montego Bay. However, according to the body of water safety, Acuacar, and the local government, the water in Cartagena is potable.
Taxis are usually plentiful but you will get a sense of that once you arrive at La Popa and whether you want to ask them comment on how much that would cost.
You should not need to worry about needing to speak Spanish to go back to the port. Most taxi drivers will understand that request in English. Try to get a map of the old city so that you don't end up wasting time walking in circles.