We had a really nice stay at Sea Spell on Grassy Key. It is beautifully landscaped, and large private property. The house is lovely and has a great view of the gulf. All of the pictures on the Vrbo website were accurate. If you have young children, I recommend this property because of the safety factor; the pool is fenced-in with latching gates, and is not directly connected to the exterior doors of the house. The upstairs screened-in porch has a staircase, which does have direct access to the pool, but there is a loud alarm on the screen door. There are no stairs inside the house; there is an elevator inside the house and a set of stairs on the outside. Again, this is a plus if you have toddlers or babies.
We did a little snorkeling and fishing off the dock while we were there. We didn't catch anything keepable, but it was fun. A couple of our family members were fascinated by the foliage on the property, and spent some time identifying what was growing. The pool and hot tub are both really nice, but if you're one to like cool water to swim in, don't go in July or August; the water was almost bath water temperature.
The beds were pretty comfortable, as was the living room furniture. The pillows, however, were not comfortable; so if you're particular about that, you might want to bring your own:-)
The ride into Marathon is about 10 minutes or so, depending on where you are going. The check in and check out process took place in Marathon, and was easier than any other time I've ever rented a property for vacation.
While there, we ate out several times; I can honestly say, all of the restaurants exceeded our expectations. Our favorites were: Keys Fisheries (outside only, but delish! And you can feed the fish), Key Colony Inn, Triton Seafood Restaurant (Italian) and Havana Jack's (short menu - more of a bar, but the food was great). We also ate at Mrs. Mac's in Key Largo on the way down. We went on a snorkeling trip through Captain Pip's; they were great! We also went to Theater of the Sea and ate at the Shrimp Shack afterward.