I've stayed at both years ago, but I took upgraded rooms. The Superior rooms at Villa Blanca were outstanding, by far much nicer than those at Casa Del Mar. Things could have changed some over the years, but compare square feet and read the details closely. Glancing on Hotels.com for August and considering Free Cancelation rates only...
Casa del Mar is listing Superior rooms much cheaper than their Standard rooms which is odd, and their Standard rooms at the same rate as their cabins so mistakes were made.
Superior room, garden view: 322 square feet @ $52/night plus substantial taxes typical of Cozumel.
Room (no name), partial view: with refrigerator, 323 square feet @ $59/night
Villa Blanca rooms all open up to the garden with a partial ocean view.
Standard: 344 square feet @ $47/night
Superior: 645 square feet @ $54/night - well worth the upgrade!
My bud & I had connecting Superior rooms. His had a couch while mine had a dining table. I think mine was the only one with a full size fridge.
So I would certainly choose the Villa Blanca Superior room, then email the hotel to request a room with a fridge. The garden there was lovely and well kept. You can book with any of many dive ops who will pick you up at either dock so you are not locked in there. You can rent tanks and weights from the onsite Op for shore dives but I wouldn't bother as it wasn't worth it at either.
Snorkeling and scuba are so different that few Ops will take both on the same boat, and even if they did - it's not a good experience. With current load restrictions in place, I doubt that anyone would take both. Traveling with non-divers is too challenging for me, but y'all can take your snorkel gear in a taxi to a beach club with good snorkeling.
You can do breakfasts at the hotel and maybe lunches nearby, but you are going to want to take a taxi into town for dinners. Always agree on a price before boarding. There are a few drivers who are not fair.