Wow, mstevens and ggunn, this turned into quite a controversy. Let me further qualify some of my comments earlier on having nothing to worry about over crime, water, and disease. I've been to Cozumel and Cancun once or twice a year over the last few years, with sporadic trips there as far back as 95. My most recent trip was in November, the one prior in March. Hey, sounds like I'm due for the next one!
On Crime: I don't read all the news reports for crime in Cancun, so I mostly speak here from just my personal travel experience. I've never had any trouble with crime in either Cancun or Cozumel nor have I ever felt in any danger while I was there. Cancun is definitely more populated, so I'd expect more crime there, and in every city there are good parts of town and bad parts of town. This said, the worst thing I've ever seen in Cancun was in October 06 when I saw some kids huffing out of a paper bag at a phone booth. I also once saw a lady pay way too much for some awful gaudy silver and beaded earrings, and if that wasn't a crime it should've been! Anyway, I'm not saying crime doesn't exist, just saying I've never been affected by it.
On Water: Different hotels have different circumstances for water purification. jbdivechic asked specifically about the Wyndham on Cozumel, and I think someone else here confirmed that Wyndham delivers purified tap water. Bottled water is always the safe way to go if you're worried about it, and the Wyndham provides bottled water, too, so no problem!
On Disease: I think you could probably make a case that you can catch something no matter where you go on planet earth. You're right, I'm not a doctor or a travel physician, nor do I have this couple's medical history, and I can't do a blood work up or give scientific recommendations on what immunizations they should or shouldn't have, but I CAN say that literally thousands of American tourists visit Cancun and Cozumel every year without getting additional immunizations and the incident rate is pretty low. You're also right that dengue and malaria can be a risk in any warm climate (malaria was only considered eradicated from the US in 1951), but the risk is hugely diminished by doing something as simple as avoiding the mosquitos and sleeping indoors at night. Also, if you check the CDC's web site on malaria risk for Mexico here's what they say:
"No malaria risk exists in the major resorts along the Pacific and Gulf coasts. Risk is very limited; therefore, prophylaxis is not recommended for most travelers to Mexico. Travelers should use personal protection measures such as insect repellents for malaria prevention." Albeit low, Dengue is the bigger risk, but there's no vaccine you can take for it, so no vaccination needed, just good sense and some bug repellent.
Again, from my personal experience, I'd say there are a lot of dangerous places in the world including plenty of parts of the good ol' USA, and by comparison Cozumel and Cancun are both pretty safe. I've never had any issues there unless I created them myself... like the time in '96 when I had too many daiquiris and climbed the flagpole in Cozumel's main square. The cops didn't find that one nearly as amusing as I did!
Luckily I got off with just a scolding, but that wasn't my best moment.
Anyway, that's it, and jbdivechic, if you're still reading this crazy thread, I hope the two of you have a wonderful trip!