...Our purpose for the trip is to just get away from the dreary Seattle winter weather and enjoy some sun, beaches, nice pool, good food, and of course, some scuba for me
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Don't like weather in Seattle? I lived out in the San Juan Islands for 6 years...
Cancun and Cozumel are quite different places. From what you say, I think Playa del Carmen south of Cancun would be ideal. Flights to Cancun are often more frequent, and cheaper, than flights to Cozumel.
Cozumel has much better reef diving than PDC. However, there is not a lot to do on Cozumel for the non-diver except shopping. The beaches are okay on the west side of the island, where most of the developments are situated. You would have to be at a resort further down the west coast of Cozumel to get fairly good beaches, and would require a taxi to get back into town.
The mainland, around PDC, has some good beaches, some okay diving (never tried it personally), and there's a lot to do for the non-diver. There is a fast (45 min) ferry service between PDC and Cozumel, so you could ride the ferry over to Cozumel for some good diving. All of the Cozumel dive ops will gladly pick you up at the ferry dock.
About an hour's drive south of PDC, near Tulum, there are a lot of cenotes that offer spectacular diving. I recommend that you avoid the cenotes unless you've master buoyancy control, the places are rather fragile.
While you're there, the Mayan ruins at Tulum are not to be missed. Less than an hour west of Tulum are the Mayan ruins of Coba. Perhaps someone can correct me if I'm wrong, but Coba is one of the only places left where you can climb to the top of a Mayan temple.
Coba is an extensive Mayan settlement, quite spread out in the jungle, you will do a lot of walking. You can ride the public bus from PDC to Tulum, but you'll need a car to get to Coba.
Don't bother with "tours," you'll spent most of your day getting ferried from one shopping area to another, and will get very little time at the actual ruins of Tulum or Coba. One "tour" of Coba was an all-day affair, and the fine print read that only 45 minutes would be spent at Coba itself!!!!! Likewise with "tours" of Tulum.
Chichen Itza is also accessible from Cancun, would make a nice day trip, though it would add significantly to your overall cost unless you drive yourself (several hours each way).
Scuba Club Cozumel is an all-inclusive dive resort on Cozumel Island. They have non-diver rates. The resort is close enough to town that you can walk there in 5 minutes. There's no beach and it's not a fancy place. Doesn't sound like it would be your kind of place, but it's the only AI place to which have been and can comment. If you return to Cozumel just to dive, SCC is the place to be!
Tulum
Climbing the temple at Coba
Tulum
Scuba Club Cozumel