I'm convinced. I have started looking at trip logistics for a trip in the spring. Thanks to everyone for your advice and encouragement.
Hyrum
Air travel & costs can be your biggest challenge, especially from there. From nearby Missoula (why does google spiel-chic object to that spelling? :silly: ) to Cozumel can be tricky and many airlines would try to make you overnight at an airport on the trip - boo on that! Fares & schedules could well change before spring, and it's best to book around 6 weeks before arrival, but many of us like to plan further out and hope the airlines don't jack with our connections too much - which they do with only token apologies.
Looking at a 6 to 8 night stay, no overnight connections, round-trip fares including taxes & fees...
March to Coz: $759 & much higher, not available on many dates!
But to Cancun: $539 to $796, with many backup flights
April to Coz: $755 & up, , not available on many dates!
To Cancun: $564 & up
May to Coz: Not available without overnight connection, and even then starts at $725!
To Canun: Any day you want, starting at $607
You may also have to do two connections each way to get the best fare, and you may prefer paying some more to do only one connection - something to shop for.
Flying in & out of Cancun is not much of a problem. It can add a few hours to your travel time each way, but you see more. There are lots of options and choices, like bus, van, private van (I use Chief's van guy, great prices on nice vans), or Maya air shuttle - but I like the van & ferry connection, maybe a meal in Playa del Carmen once you check your bags in on the ferry (but don't miss that boat then!), and mostly avoiding the sardine packed shared vans at the Coz airport which you'd still have to do if you took the air shuttle.
The cautions of flying in & out of Cancun are mostly arriving early enough to make the connections to the island ok (last time I was delayed in Texas, then spent hours at my dentist office, and barely made the last shuttle!), as well as getting back to the Cancun airport for the required check-in without catching a sunrise ferry.
I'd need to know more about your preferences to do a better job, but let's look at March for 8 nights so you can skip a dive day or two for rest & other tourism and still get several dive days. $539 with 2 connections and 11-13 hours of flight time each way vs. $657 for only 1 connection and 7-11 hours of flights each way? I'd spend the extra and cut down on time & hassle. This one looks good to me, leaving Montana at 7am, arriving Cancun at 4pm, clearing I&M by 5 or 6, into Playa del Carmen for dinner, a 30-45 minute ferry ride, taxi to your hotel mid evening to set up camp in your room. On your return, a late morning ferry, lunch in PDC, and still get to the airport plenty early for a 2:50pm flight, plenty of time in Denver for I&C (airline often don't allow enough on their plans), back in Montana around midnight.

But that's just one plan. You have lots of time to work over various approaches and do it your way...