After some research, here is what I've found. Sharing it here in case others find this useful in the future:
1: State Department
For American citizens, the State Department publishes a Traveler's Checklist which contains general advice for preparing for a trip abroad.
In addition, they also post travel advisories for other countries, including a very detailed list of Mexican states, with current advisory levels ranging from "Exercise Increased Caution" in the area near Cancun, up to "Do Not Travel" for the 5 states most heavily affected by cartel violence.
One of the recommended steps for travelers to take is to keep a copy of the address and contact info of nearby embassies/consuls. For the U.S., a list a Consular agencies in the Merida region near Cancun can be found here.
2: STEP Program
The State Department also recommends enrolling in the Smart Traveler Enrollment Program, which lets you register emergency contacts for yourself, and specify the location, reason, duration, and emergency contacts for your travels with the State Department.
3: Center for Disease Control
I spoke to my doctor about vaccines, and although he said no vaccines were needed to travel to Mexico, he recommended checking the CDC's website to see health warnings before traveling to any new destination. The CDC's list of health information for traveling in Mexico is here.
For travelers from other nations, I'm sure that each country's government provides similar resources to those listed above. So be sure to check with your own government to find out details like embassy information etc.
Overall the takeaway from my research is that the area near Cancun is relatively safe, although 2017 saw an uptick in murders there, including some incidents in areas frequented by tourists in which bystanders were injured. But most of the areas that are deemed dangerous by the State Department are near the borders, and along the Pacific coast. Tourist centers are not often a focal point for organized crime, just the odd pickpocketing or robbery that you might have in any big city.
Addendum from after my trip
Overall, I didn't find many places that I felt unsafe. My friends and I stayed in a small apartment in central Cancún and either walked or took buses almost everywhere. Pedestrian traffic was very common in all parts of Cancún that we visited. The buildings are mostly surrounded by gates/walls, often with electrified cable or barbed wire on top, but outside of that I didn't see any indication of crime during the daytime or early evening when we were out.
We also visited Tulum, which is a much smaller/poorer city. There were signs of poverty on the town's outskirts, but not much that spoke of actual crime. Again, tons of pedestrian traffic in the city center, and we even hitchhiked (as a group) with a young guy when heading to the Mayan ruins on the coast.
Overall, the areas I visited were quite nice, and seemed very safe, at least during the times a tourist would be visiting. A few parting tips though:
- If you leave the very touristy areas, plan to speak Spanish. Many service industry professionals in central Cancún are not bilingual - tourists don't visit those areas much, so you're not their target audience.
- Even if your Spanish is bad, most people appreciate you making the effort and will bear with your bad grammar/accent.
- Taxis at the bus station cost double what they cost if you hail them roadside.
- The dollar symbol ($) is also used for pesos. Prices are rarely listed in USD, but when they are, it will be indicated. Ask if you're unsure.
- The tap water isn't good to drink. But in the area we stayed, water vendors visited 6 days per week selling 20-liter drums of water for 32 pesos (About $1.50 US.) Listen for them shouting "¡Aguaaaa!"
- Expect European/American prices for food/drink in the Hotel Zone (beach resort area.) You can get the same stuff in the Cancún city center for 30-50% the price.
- Most places accept credit cards. But street vendors and taxi drivers don't, so carry a little bit of cash (I tried to carry about 500 pesos, which is about $25 US)
- If you drink, give Mezcal a try. It's like a cross between Tequila and Scotch. I found it much to my liking.
- In-home washing machines are very rare in Cancún. Instead, most people rely on Lavanderías, which are all over the place. It's basically what we call a "wash-and-fold" in the U.S. You pay by the kilogram. A week's worth of clothes cost me about $5 US to launder.