Key Colony Beach is a wonderful place to stay in Marathon. This unit was tastefully updated/decorated and fit the bill perfectly for our Keys vacation. It's right on the canal (we saw a mom and baby manatee the first day we got there), and it's close to anything you need in Marathon and the Keys in general. We used the KCB club one day, ate at the restaurant one night, and enjoyed kayaking and snorkeling with some local guides. The unit has good A/C, all the amenities you need for a stay, nice TV's (although who goes to the Keys to watch Netflix?), and a well supplied kitchen and gas grill (although...please clean it when you're done...the last guest didn't so I had to clean it before use. Don't be "that guy".)
I would highly recommend this unit and I would book here again in a heartbeat.